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5 Database Administration

This chapter covers topics that deal with administering a MySQL installation, such as configuring the server, managing user accounts, and performing backups.

5.1 The MySQL Server and Server Startup Scripts

The MySQL server, mysqld, is the main program that does most of the work in a MySQL installation. The server is accompanied by several related scripts that perform setup operations when you install MySQL or that are helper programs to assist you in starting and stopping the server.

This section provides an overview of the server and related programs, and information about server startup scripts. Information about configuring the server itself is given in section 5.2 Configuring the MySQL Server.

5.1.1 Overview of the Server-Side Scripts and Utilities

All MySQL programs take many different options. However, every MySQL program provides a --help option that you can use to get a description of the program's options. For example, try mysqld --help.

You can override default options for all standard programs by specifying options on the command line or in an option file. section 4.3 Specifying Program Options.

The following list briefly describes the MySQL server and server-related programs:

mysqld
The SQL daemon (that is, the MySQL server). To use client programs, this program must be running, because clients gain access to databases by connecting to the server. See section 5.2 Configuring the MySQL Server.
mysqld-max
A version of the server that includes additional features. See section 5.1.2 The mysqld-max Extended MySQL Server.
mysqld_safe
A server startup script. mysqld_safe attempts to start mysqld-max if it exists, and mysqld otherwise. See section 5.1.3 The mysqld_safe Server Startup Script.
mysql.server
A server startup script. This script is used on systems that use run directories containing scripts that start system services for particular run levels. It invokes mysqld_safe to start the MySQL server. See section 5.1.4 The mysql.server Server Startup Script.
mysqld_multi
A server startup script that can start or stop multiple servers installed on the system. See section 5.1.5 The mysqld_multi Program for Managing Multiple MySQL Servers.
mysql_install_db
This script creates the MySQL grant tables with default privileges. It is usually executed only once, when first installing MySQL on a system.
mysql_fix_privilege_tables
This script is used after an upgrade install operation, to update the grant tables with any changes that have been made in newer versions of MySQL.

There are several other programs that also are run on the server host:

myisamchk
A utility to describe, check, optimize, and repair MyISAM tables. myisamchk is described in section 5.7.2 Table Maintenance and Crash Recovery.
make_binary_distribution
This program makes a binary release of a compiled MySQL. This could be sent by FTP to `/pub/mysql/upload/' on ftp.mysql.com for the convenience of other MySQL users.
mysqlbug
The MySQL bug reporting script. It can be used to send a bug report to the MySQL mailing list. (You can also visit http://bugs.mysql.com/ to file a bug report online.)

5.1.2 The mysqld-max Extended MySQL Server

A MySQL-Max server is a version of the mysqld MySQL server that has been built to include additional features.

The distribution to use depends on your platform:

You can find the MySQL-Max binaries on the MySQL AB Web site at http://dev.mysql.com/downloads/mysql-4.0.html.

MySQL AB builds the MySQL-Max servers by using the following configure options:

--with-server-suffix=-max
This option adds a -max suffix to the mysqld version string.
--with-innodb
This option enables support for the InnoDB storage engine. MySQL-Max servers always include InnoDB support, but this option actually is needed only for MySQL 3.23. From MySQL 4 on, InnoDB is included by default in binary distributions, so you do not need a MySQL-Max server to obtain InnoDB support.
--with-bdb
This option enables support for the Berkeley DB (BDB) storage engine.
CFLAGS=-DUSE_SYMDIR
This define enables symbolic link support for Windows.

MySQL-Max binary distributions are a convenience for those who wish to install precompiled programs. If you build MySQL using a source distribution, you can build your own Max-like server by enabling the same features at configuration time that the MySQL-Max binary distributions are built with.

MySQL-Max servers include the BerkeleyDB (BDB) storage engine whenever possible, but not all platforms support BDB. The following table shows which platforms allow MySQL-Max binaries to include BDB:

System BDB Support
AIX 4.3 N
HP-UX 11.0 N
Linux-Alpha N
Linux-IA-64 N
Linux-Intel Y
Mac OS X N
NetWare N
SCO OSR5 Y
Solaris-Intel N
Solaris-SPARC Y
UnixWare Y
Windows/NT Y

To find out which storage engines your server supports, issue the following statement:

mysql> SHOW ENGINES;

Before MySQL 4.1.2, SHOW ENGINES is unavailable. Use the following statement instead and check the value of the variable for the storage engine in which you are interested:

mysql> SHOW VARIABLES LIKE 'have_%';
+------------------+----------+
| Variable_name    | Value    |
+------------------+----------+
| have_bdb         | NO       |
| have_crypt       | YES      |
| have_innodb      | YES      |
| have_isam        | NO       |
| have_raid        | NO       |
| have_symlink     | DISABLED |
| have_openssl     | NO       |
| have_query_cache | YES      |
+------------------+----------+

The values in the second column indicate the server's level of support for each feature:

Value Meaning
YES The feature is supported and is active.
NO The feature is not supported.
DISABLED The feature is supported but has been disabled.

A value of NO means that the server was compiled without support for the feature, so it cannot be activated at runtime.

A value of DISABLED occurs either because the server was started with an option that disables the feature, or because not all options required to enable it were given. In the latter case, the host_name.err error log file should contain a reason indicating why the option is disabled.

One situation in which you might see DISABLED occurs with MySQL 3.23 when the InnoDB storage engine is compiled in. In MySQL 3.23, you must supply at least the innodb_data_file_path option at runtime to set up the InnoDB tablespace. Without this option, InnoDB disables itself. See section 15.3 InnoDB in MySQL 3.23. You can specify configuration options for the BDB storage engine, too, but BDB will not disable itself if you do not provide them. See section 14.4.3 BDB Startup Options.

You might also see DISABLED for the InnoDB, BDB, or ISAM storage engines if the server was compiled to support them, but was started with the --skip-innodb, --skip-bdb, or --skip-isam options at runtime.

As of Version 3.23, all MySQL servers support MyISAM tables, because MyISAM is the default storage engine.

5.1.3 The mysqld_safe Server Startup Script

mysqld_safe is the recommended way to start a mysqld server on Unix and NetWare. mysqld_safe adds some safety features such as restarting the server when an error occurs and logging runtime information to an error log file. NetWare-specific behaviors are listed later in this section.

Note: Before MySQL 4.0, mysqld_safe is named safe_mysqld. To preserve backward compatibility, MySQL binary distributions for some time will include safe_mysqld as a symbolic link to mysqld_safe.

By default, mysqld_safe tries to start an executable named mysqld-max if it exists, or mysqld otherwise. Be aware of the implications of this behavior:

To override the default behavior and specify explicitly which server you want to run, specify a --mysqld or --mysqld-version option to mysqld_safe.

Many of the options to mysqld_safe are the same as the options to mysqld. See section 5.2.1 mysqld Command-Line Options.

All options specified to mysqld_safe on the command line are passed to mysqld. If you want to use any options that are specific to mysqld_safe and that mysqld doesn't support, do not specify them on the command line. Instead, list them in the [mysqld_safe] group of an option file. See section 4.3.2 Using Option Files.

mysqld_safe reads all options from the [mysqld], [server], and [mysqld_safe] sections in option files. For backward compatibility, it also reads [safe_mysqld] sections, although you should rename such sections to [mysqld_safe] when you begin using MySQL 4.0 or later.

mysqld_safe supports the following options:

--basedir=path
The path to the MySQL installation directory.
--core-file-size=size
The size of the core file mysqld should be able to create. The option value is passed to ulimit -c.
--datadir=path
The path to the data directory.
--defaults-extra-file=path
The name of an option file to be read in addition to the usual option files.
--defaults-file=path
The name of an option file to be read instead of the usual option files.
--err-log=path
The old form of the --log-error option, to be used before MySQL 4.0.
--ledir=path
The path to the directory containing the mysqld program. Use this option to explicitly indicate the location of the server.
--log-error=path
Write the error log to the given file. See section 5.9.1 The Error Log.
--mysqld=prog_name
The name of the server program (in the ledir directory) that you want to start. This option is needed if you use the MySQL binary distribution but have the data directory outside of the binary distribution.
--mysqld-version=suffix
This option is similar to the --mysqld option, but you specify only the suffix for the server program name. The basename is assumed to be mysqld. For example, if you use --mysqld-version=max, mysqld_safe will start the mysqld-max program in the ledir directory. If the argument to --mysqld-version is empty, mysqld_safe uses mysqld in the ledir directory.
--nice=priority
Use the nice program to set the server's scheduling priority to the given value. This option was added in MySQL 4.0.14.
--no-defaults
Do not read any option files.
--open-files-limit=count
The number of files mysqld should be able to open. The option value is passed to ulimit -n. Note that you need to start mysqld_safe as root for this to work properly!
--pid-file=path
The path to the process ID file.
--port=port_num
The port number to use when listening for TCP/IP connections.
--socket=path
The Unix socket file to use for local connections.
--timezone=zone
Set the TZ time zone environment variable to the given option value. Consult your operating system documentation for legal time zone specification formats.
--user={user_name | user_id}
Run the mysqld server as the user having the name user_name or the numeric user ID user_id. (``User'' in this context refers to a system login account, not a MySQL user listed in the grant tables.)

The mysqld_safe script is written so that it normally can start a server that was installed from either a source or a binary distribution of MySQL, even though these types of distributions typically install the server in slightly different locations. (See section 2.1.5 Installation Layouts.) mysqld_safe expects one of the following conditions to be true:

Because mysqld_safe will try to find the server and databases relative to its own working directory, you can install a binary distribution of MySQL anywhere, as long as you run mysqld_safe from the MySQL installation directory:

shell> cd mysql_installation_directory
shell> bin/mysqld_safe &

If mysqld_safe fails, even when invoked from the MySQL installation directory, you can specify the --ledir and --datadir options to indicate the directories in which the server and databases are located on your system.

Normally, you should not edit the mysqld_safe script. Instead, configure mysqld_safe by using command-line options or options in the [mysqld_safe] section of a `my.cnf' option file. In rare cases, it might be necessary to edit mysqld_safe to get it to start the server properly. However, if you do this, your modified version of mysqld_safe might be overwritten if you upgrade MySQL in the future, so you should make a copy of your edited version that you can reinstall.

On NetWare, mysqld_safe is a NetWare Loadable Module (NLM) that is ported from the original Unix shell script. It does the following:

  1. Runs a number of system and option checks.
  2. Runs a check on MyISAM and ISAM tables.
  3. Provides a screen presence for the MySQL server.
  4. Starts mysqld, monitors it, and restarts it if it terminates in error.
  5. Sends error messages from mysqld to the `host_name.err' file in the data directory.
  6. Sends mysqld_safe screen output to the `host_name.safe' file in the data directory.

5.1.4 The mysql.server Server Startup Script

MySQL distributions on Unix include a script named mysql.server. It can be used on systems such as Linux and Solaris that use System V-style run directories to start and stop system services. It is also used by the Mac OS X Startup Item for MySQL.

mysql.server can be found in the `support-files' directory under your MySQL installation directory or in a MySQL source tree.

If you use the Linux server RPM package (MySQL-server-VERSION.rpm), the mysql.server script will already have been installed in the `/etc/init.d' directory with the name `mysql'. You need not install it manually. See section 2.4 Installing MySQL on Linux for more information on the Linux RPM packages.

Some vendors provide RPM packages that install a startup script under a different name such as mysqld.

If you install MySQL from a source distribution or using a binary distribution format that does not install mysql.server automatically, you can install it manually. Instructions are provided in section 2.9.2.2 Starting and Stopping MySQL Automatically.

mysql.server reads options from the [mysql.server] and [mysqld] sections of option files. (For backward compatibility, it also reads [mysql_server] sections, although you should rename such sections to [mysql.server] when you begin using MySQL 4.0 or later.)

5.1.5 The mysqld_multi Program for Managing Multiple MySQL Servers

mysqld_multi is meant for managing several mysqld processes that listen for connections on different Unix socket files and TCP/IP ports. It can start or stop servers, or report their current status.

The program searches for groups named [mysqld#] in `my.cnf' (or in the file named by the --config-file option). # can be any positive integer. This number is referred to in the following discussion as the option group number, or GNR. Group numbers distinguish option groups from one another and are used as arguments to mysqld_multi to specify which servers you want to start, stop, or obtain a status report for. Options listed in these groups are the same that you would use in the [mysqld] group used for starting mysqld. (See, for example, section 2.9.2.2 Starting and Stopping MySQL Automatically.) However, when using multiple servers it is necessary that each one use its own value for options such as the Unix socket file and TCP/IP port number. For more information on which options must be unique per server in a multiple-server environment, see section 5.10 Running Multiple MySQL Servers on the Same Machine.

To invoke mysqld_multi, use the following syntax:

shell> mysqld_multi [options] {start|stop|report} [GNR[,GNR]...]

start, stop, and report indicate which operation you want to perform. You can perform the designated operation on a single server or multiple servers, depending on the GNR list that follows the option name. If there is no list, mysqld_multi performs the operation for all servers in the option file.

Each GNR value represents an option group number or range of group numbers. The value should be the number at the end of the group name in the option file. For example, the GNR for a group named [mysqld17] is 17. To specify a range of numbers, separate the first and last numbers by a dash. The GNR value 10-13 represents groups [mysqld10] through [mysqld13]. Multiple groups or group ranges can be specified on the command line, separated by commas. There must be no whitespace characters (spaces or tabs) in the GNR list; anything after a whitespace character is ignored.

This command starts a single server using option group [mysqld17]:

shell> mysqld_multi start 17

This command stops several servers, using option groups [mysql8] and [mysqld10] through [mysqld13]:

shell> mysqld_multi stop 8,10-13

For an example of how you might set up an option file, use this command:

shell> mysqld_multi --example

mysqld_multi supports the following options:

--config-file=name
Specify the name of an alternative option file. This affects where mysqld_multi looks for [mysqld#] option groups. Without this option, all options are read from the usual `my.cnf' file. The option does not affect where mysqld_multi reads its own options, which are always taken from the [mysqld_multi] group in the usual `my.cnf' file.
--example
Display a sample option file.
--help
Display a help message and exit.
--log=name
Specify the name of the log file. If the file exists, log output is appended to it.
--mysqladmin=prog_name
The mysqladmin binary to be used to stop servers.
--mysqld=prog_name
The mysqld binary to be used. Note that you can specify mysqld_safe as the value for this option also. The options are passed to mysqld. Just make sure that you have the directory where mysqld is located in your PATH environment variable setting or fix mysqld_safe.
--no-log
Print log information to stdout rather than to the log file. By default, output goes to the log file.
--password=password
The password of the MySQL account to use when invoking mysqladmin. Note that the password value is not optional for this option, unlike for other MySQL programs.
--silent
Disable warnings. This option was added in MySQL 4.1.6.
--tcp-ip
Connect to each MySQL server via the TCP/IP port instead of the Unix socket file. (If a socket file is missing, the server might still be running, but accessible only via the TCP/IP port.) By default, connections are made using the Unix socket file. This option affects stop and report operations.
--user=user_name
The username of the MySQL account to use when invoking mysqladmin.
--verbose
Be more verbose. This option was added in MySQL 4.1.6.
--version
Display version information and exit.

Some notes about mysqld_multi:

The following example shows how you might set up an option file for use with mysqld_multi. The first and fifth [mysqld#] group were intentionally left out from the example to illustrate that you can have ``gaps'' in the option file. This gives you more flexibility. The order in which the mysqld programs are started or stopped depends on the order in which they appear in the option file.

# This file should probably be in your home dir (~/.my.cnf)
# or /etc/my.cnf
# Version 2.1 by Jani Tolonen

[mysqld_multi]
mysqld     = /usr/local/bin/mysqld_safe
mysqladmin = /usr/local/bin/mysqladmin
user       = multi_admin
password   = multipass

[mysqld2]
socket     = /tmp/mysql.sock2
port       = 3307
pid-file   = /usr/local/mysql/var2/hostname.pid2
datadir    = /usr/local/mysql/var2
language   = /usr/local/share/mysql/english
user       = john

[mysqld3]
socket     = /tmp/mysql.sock3
port       = 3308
pid-file   = /usr/local/mysql/var3/hostname.pid3
datadir    = /usr/local/mysql/var3
language   = /usr/local/share/mysql/swedish
user       = monty

[mysqld4]
socket     = /tmp/mysql.sock4
port       = 3309
pid-file   = /usr/local/mysql/var4/hostname.pid4
datadir    = /usr/local/mysql/var4
language   = /usr/local/share/mysql/estonia
user       = tonu

[mysqld6]
socket     = /tmp/mysql.sock6
port       = 3311
pid-file   = /usr/local/mysql/var6/hostname.pid6
datadir    = /usr/local/mysql/var6
language   = /usr/local/share/mysql/japanese
user       = jani

See section 4.3.2 Using Option Files.

5.2 Configuring the MySQL Server

This section discusses MySQL server configuration topics:

5.2.1 mysqld Command-Line Options

When you start the mysqld server, you can specify program options using any of the methods described in section 4.3 Specifying Program Options. The most common methods are to provide options in an option file or on the command line. However, in most cases it is desirable to make sure that the server uses the same options each time it runs. The best way to ensure this is to list them in an option file. See section 4.3.2 Using Option Files.

mysqld reads options from the [mysqld] and [server] groups. mysqld_safe reads options from the [mysqld], [server], [mysqld_safe], and [safe_mysqld] groups. mysql.server reads options from the [mysqld] and [mysql.server] groups. An embedded MySQL server usually reads options from the [server], [embedded], and [xxxxx_SERVER] groups, where xxxxx is the name of the application into which the server is embedded.

mysqld accepts many command-line options. For a list, execute mysqld --help. Before MySQL 4.1.1, --help prints the full help message. As of 4.1.1, it prints a brief message; to see the full list, use mysqld --verbose --help.

The following list shows some of the most common server options. Additional options are described elsewhere:

You can also set the value of a server system variable by using the variable name as an option, as described later in this section.

--help, -?
Display a short help message and exit. Before MySQL 4.1.1, --help displays the full help message. As of 4.1.1, it displays an abbreviated message only. Use both the --verbose and --help options to see the full message.
--ansi
Use standard SQL syntax instead of MySQL syntax. See section 1.5.3 Running MySQL in ANSI Mode. For more precise control over the server SQL mode, use the --sql-mode option instead.
--basedir=path, -b path
The path to the MySQL installation directory. All paths are usually resolved relative to this.
--big-tables
Allow large result sets by saving all temporary sets in files. This option prevents most ``table full'' errors, but also slows down queries for which in-memory tables would suffice. Since MySQL 3.23.2, the server is able to handle large result sets automatically by using memory for small temporary tables and switching to disk tables where necessary.
--bind-address=IP
The IP address to bind to.
--console
Write the error log messages to stderr/stdout even if --log-error is specified. On Windows, mysqld will not close the console screen if this option is used.
--character-sets-dir=path
The directory where character sets are installed. See section 5.8.1 The Character Set Used for Data and Sorting.
--chroot=path
Put the mysqld server in a closed environment during startup by using the chroot() system call. This is a recommended security measure as of MySQL 4.0. (MySQL 3.23 is not able to provide a chroot() jail that is 100% closed.) Note that use of this option somewhat limits LOAD DATA INFILE and SELECT ... INTO OUTFILE.
--character-set-server=charset
Use charset as the default server character set. This option is available as of MySQL 4.1.3. See section 5.8.1 The Character Set Used for Data and Sorting.
--core-file
Write a core file if mysqld dies. For some systems, you must also specify the --core-file-size option to mysqld_safe. See section 5.1.3 The mysqld_safe Server Startup Script. Note that on some systems, such as Solaris, you will not get a core file if you are also using the --user option.
--collation-server=collation
Use collation as the default server collation. This option is available as of MySQL 4.1.3. See section 5.8.1 The Character Set Used for Data and Sorting.
--datadir=path, -h path
The path to the data directory.
--debug[=debug_options], -# [debug_options]
If MySQL is configured with --with-debug, you can use this option to get a trace file of what mysqld is doing. The debug_options string often is 'd:t:o,file_name'. See section E.1.2 Creating Trace Files.
--default-character-set=charset
Use charset as the default character set. This option is deprecated in favor of --character-set-server as of MySQL 4.1.3. See section 5.8.1 The Character Set Used for Data and Sorting.
--default-collation=collation
Use collation as the default collation. This option is deprecated in favor of --collation-server as of MySQL 4.1.3. See section 5.8.1 The Character Set Used for Data and Sorting.
--default-storage-engine=type
This option is a synonym for --default-table-type. It is available as of MySQL 4.1.2.
--default-table-type=type
Set the default table type for tables. See section 14 MySQL Storage Engines and Table Types.
--default-time-zone=type
Set the default server time zone. This option sets the global time_zone system variable. If this option is not given, the default time zone will be the same as the system time zone (given by the value of the system_time_zone system variable. This option is available as of MySQL 4.1.3.
--delay-key-write[= OFF | ON | ALL]
How the DELAYED KEYS option should be used. Delayed key writing causes key buffers not to be flushed between writes for MyISAM tables. OFF disables delayed key writes. ON enables delayed key writes for those tables that were created with the DELAYED KEYS option. ALL delays key writes for all MyISAM tables. Available as of MySQL 4.0.3. See section 7.5.2 Tuning Server Parameters. See section 14.1.1 MyISAM Startup Options. Note: If you set this variable to ALL, you should not use MyISAM tables from within another program (such as from another MySQL server or with myisamchk) when the table is in use. Doing so will lead to index corruption.
--delay-key-write-for-all-tables
Old form of --delay-key-write=ALL for use prior to MySQL 4.0.3. As of 4.0.3, use --delay-key-write instead.
--des-key-file=file_name
Read the default keys used by DES_ENCRYPT() and DES_DECRYPT() from this file.
--enable-named-pipe
Enable support for named pipes. This option applies only on Windows NT, 2000, XP, and 2003 systems, and can be used only with the mysqld-nt and mysqld-max-nt servers that support named pipe connections.
--exit-info[=flags], -T [flags]
This is a bit mask of different flags you can use for debugging the mysqld server. Do not use this option unless you know exactly what it does!
--external-locking
Enable system locking. Note that if you use this option on a system on which lockd does not fully work (as on Linux), you will easily get mysqld to deadlock. This option previously was named --enable-locking. Note: If you use this option to enable updates to MyISAM tables from many MySQL processes, you have to ensure that these conditions are satisfied: The easiest way to ensure this is to always use --external-locking together with --delay-key-write=OFF --query-cache-size=0. (This is not done by default because in many setups it's useful to have a mixture of the above options.)
--flush
Flush all changes to disk after each SQL statement. Normally MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synching to disk. See section A.4.2 What to Do If MySQL Keeps Crashing.
--init-file=file
Read SQL statements from this file at startup. Each statement must be on a single line and should not include comments.
--innodb-safe-binlog
Adds consistency guarantees between the content of InnoDB tables and the binary log. See section 5.9.4 The Binary Log.
--language=lang_name, -L lang_name
Client error messages in given language. lang_name can be given as the language name or as the full pathname to the directory where the language files are installed. See section 5.8.2 Setting the Error Message Language.
--log[=file], -l [file]
Log connections and queries to this file. See section 5.9.2 The General Query Log. If you don't specify a filename, MySQL will use host_name.log as the filename.
--log-bin=[file]
The binary log file. Log all queries that change data to this file. Used for backup and replication. See section 5.9.4 The Binary Log. If you don't specify a filename, MySQL will use host_name-bin as the log file basename.
--log-bin-index[=file]
The index file for binary log filenames. See section 5.9.4 The Binary Log. If you don't specify a filename, MySQL will use host_name-bin.index as the filename.
--log-error[=file]
Log errors and startup messages to this file. See section 5.9.1 The Error Log. If you don't specify a filename, MySQL will use host_name.err as the filename.
--log-isam[=file]
Log all ISAM/MyISAM changes to this file (used only when debugging ISAM/MyISAM).
--log-long-format
Log some extra information to the log files (update log, binary update log, and slow queries log, whatever log has been activated). For example, username and timestamp are logged for queries. If you are using --log-slow-queries and --log-long-format, queries that are not using indexes also are logged to the slow query log. Note that --log-long-format is deprecated as of MySQL version 4.1, when --log-short-format was introduced (the long log format is the default setting since version 4.1). Also note that starting with MySQL 4.1, the --log-queries-not-using-indexes option is available for the purpose of logging queries that do not use indexes to the slow query log.
--log-queries-not-using-indexes
If you are using this option with --log-slow-queries, then queries that are not using indexes also are logged to the slow query log. This option is available as of MySQL 4.1. See section 5.9.5 The Slow Query Log.
--log-short-format
Log less information to the log files (update log, binary update log, and slow queries log, whatever log has been activated). For example, username and timestamp are not logged for queries. This option was introduced in MySQL 4.1.
--log-slow-queries[=file]
Log all queries that have taken more than long_query_time seconds to execute to this file. See section 5.9.5 The Slow Query Log. Note that the default for the amount of information logged has changed in MySQL 4.1. See the --log-long-format and --log-short-format options for details.
--log-update[=file]
Log updates to file# where # is a unique number if not given. See section 5.9.3 The Update Log. The update log is deprecated and is removed in MySQL 5.0.0; you should use the binary log instead (--log-bin). See section 5.9.4 The Binary Log. Starting from version 5.0.0, using --log-update will just turn on the binary log instead (see section D.1.4 Changes in release 5.0.0 (22 Dec 2003: Alpha)).
--log-warnings, -W
Print out warnings such as Aborted connection... to the error log. Enabling this option is recommended, for example, if you use replication (you will get more information about what is happening, such as messages about network failures and reconnections). This option is enabled by default as of MySQL 4.0.19 and 4.1.2; to disable it, use --skip-log-warnings. As of MySQL 4.0.21 and 4.1.3, aborted connections are not logged to the error log unless the value is greater than 1. See section A.2.10 Communication Errors and Aborted Connections. This option was named --warnings before MySQL 4.0.
--low-priority-updates
Table-modifying operations (INSERT, REPLACE, DELETE, UPDATE) will have lower priority than selects. This can also be done via {INSERT | REPLACE | DELETE | UPDATE} LOW_PRIORITY ... to lower the priority of only one query, or by SET LOW_PRIORITY_UPDATES=1 to change the priority in one thread. See section 7.3.2 Table Locking Issues.
--memlock
Lock the mysqld process in memory. This works on systems such as Solaris that support the mlockall() system call. This might help if you have a problem where the operating system is causing mysqld to swap on disk. Note that use of this option requires that you run the server as root, which is normally not a good idea for security reasons.
--myisam-recover [=option[,option...]]]
Set the MyISAM storage engine recovery mode. The option value is any combination of the values of DEFAULT, BACKUP, FORCE, or QUICK. If you specify multiple values, separate them by commas. You can also use a value of "" to disable this option. If this option is used, mysqld will, when it opens a MyISAM table, open check whether the table is marked as crashed or wasn't closed properly. (The last option works only if you are running with --skip-external-locking.) If this is the case, mysqld will run a check on the table. If the table was corrupted, mysqld will attempt to repair it. The following options affect how the repair works:
Option Description
DEFAULT The same as not giving any option to --myisam-recover.
BACKUP If the data file was changed during recovery, save a backup of the `tbl_name.MYD' file as `tbl_name-datetime.BAK'.
FORCE Run recovery even if we will lose more than one row from the `.MYD' file.
QUICK Don't check the rows in the table if there aren't any delete blocks.
Before a table is automatically repaired, MySQL will add a note about this in the error log. If you want to be able to recover from most problems without user intervention, you should use the options BACKUP,FORCE. This will force a repair of a table even if some rows would be deleted, but it will keep the old data file as a backup so that you can later examine what happened. This option is available as of MySQL 3.23.25.
--ndb-connectstring=connect_string
When using the NDB storage engine, it is possible to point out the management server that distributes the cluster configuration by setting the connect string option. See section 16.3.4.2 The MySQL Cluster connectstring for syntax.
--ndbcluster
If the binary includes support for the NDB Cluster storage engine (from version 4.1.3, the MySQL-Max binaries are built with NDB Cluster enabled) the default disabling of support for the NDB Cluster storage engine can be overruled by using this option. Using the NDB Cluster storage engine is necessary for using MySQL Cluster. See section 16 MySQL Cluster.
--new
The --new option can be used to make the server behave as 4.1 in certain respects, easing a 4.0 to 4.1 upgrade: This option can be used to help you see how your applications will behave in MySQL 4.1, without actually upgrading to 4.1.
--pid-file=path
The path to the process ID file used by mysqld_safe.
--port=port_num, -P port_num
The port number to use when listening for TCP/IP connections.
--old-protocol, -o
Use the 3.20 protocol for compatibility with some very old clients. See section 2.10.6 Upgrading from Version 3.20 to 3.21.
--one-thread
Only use one thread (for debugging under Linux). This option is available only if the server is built with debugging enabled. See section E.1 Debugging a MySQL Server.
--open-files-limit=count
To change the number of file descriptors available to mysqld. If this is not set or set to 0, then mysqld will use this value to reserve file descriptors to use with setrlimit(). If this value is 0 then mysqld will reserve max_connections*5 or max_connections + table_cache*2 (whichever is larger) number of files. You should try increasing this if mysqld gives you the error "Too many open files."
--safe-mode
Skip some optimization stages.
--safe-show-database
With this option, the SHOW DATABASES statement displays only the names of those databases for which the user has some kind of privilege. As of MySQL 4.0.2, this option is deprecated and doesn't do anything (it is enabled by default), because there is now a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis. See section 5.5.3 Privileges Provided by MySQL.
--safe-user-create
If this is enabled, a user can't create new users with the GRANT statement, if the user doesn't have the INSERT privilege for the mysql.user table or any column in the table.
--secure-auth
Disallow authentication for accounts that have old (pre-4.1) passwords. This option is available as of MySQL 4.1.1.
--shared-memory
Enable shared-memory connections by local clients. This option is available only on Windows. It was added in MySQL 4.1.0.
--shared-memory-base-name=name
The name to use for shared-memory connections. This option is available only on Windows. It was added in MySQL 4.1.0.
--skip-bdb
Disable the BDB storage engine. This saves memory and might speed up some operations. Do not use this option if you require BDB tables.
--skip-concurrent-insert
Turn off the ability to select and insert at the same time on MyISAM tables. (This is to be used only if you think you have found a bug in this feature.)
--skip-delay-key-write
Ignore the DELAY_KEY_WRITE option for all tables. As of MySQL 4.0.3, you should use --delay-key-write=OFF instead. See section 7.5.2 Tuning Server Parameters.
--skip-external-locking
Don't use system locking. To use isamchk or myisamchk, you must shut down the server. See section 1.2.3 MySQL Stability. In MySQL 3.23, you can use CHECK TABLE and REPAIR TABLE to check and repair MyISAM tables. This option previously was named --skip-locking.
--skip-grant-tables
This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing a mysqladmin flush-privileges or mysqladmin reload command, or by issuing a FLUSH PRIVILEGES statement.)
--skip-host-cache
Do not use the internal hostname cache for faster name-to-IP resolution. Instead, query the DNS server every time a client connects. See section 7.5.6 How MySQL Uses DNS.
--skip-innodb
Disable the InnoDB storage engine. This saves memory and disk space and might speed up some operations. Do not use this option if you require InnoDB tables.
--skip-isam
Disable the ISAM storage engine. As of MySQL 4.1, ISAM is disabled by default, so this option applies only if the server was configured with support for ISAM. This option was added in MySQL 4.1.1.
--skip-name-resolve
Do not resolve hostnames when checking client connections. Use only IP numbers. If you use this option, all Host column values in the grant tables must be IP numbers or localhost. See section 7.5.6 How MySQL Uses DNS.
--skip-ndbcluster
Disable the NDB Cluster storage engine. This is the default for binaries that were built with NDB Cluster storage engine support, this means that the system will only allocate memory and other resources for this storage engine if it is explicitly enabled.
--skip-networking
Don't listen for TCP/IP connections at all. All interaction with mysqld must be made via named pipes or shared memory (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are allowed. See section 7.5.6 How MySQL Uses DNS.
--skip-new
Don't use new, possibly wrong routines.
--skip-symlink
This is the old form of --skip-symbolic-links, for use before MySQL 4.0.13.
--symbolic-links, --skip-symbolic-links
Enable or disable symbolic link support. This option has different effects on Windows and Unix: This option was added in MySQL 4.0.13.
--skip-safemalloc
If MySQL is configured with --with-debug=full, all MySQL programs check for memory overruns during each memory allocation and memory freeing operation. This checking is very slow, so for the server you can avoid it when you don't need it by using the --skip-safemalloc option.
--skip-show-database
With this option, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. Without this option, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database.
--skip-stack-trace
Don't write stack traces. This option is useful when you are running mysqld under a debugger. On some systems, you also must use this option to get a core file. See section E.1 Debugging a MySQL Server.
--skip-thread-priority
Disable using thread priorities for faster response time.
--socket=path
On Unix, this option specifies the Unix socket file to use for local connections. The default value is `/tmp/mysql.sock'. On Windows, the option specifies the pipe name to use for local connections that use a named pipe. The default value is MySQL.
--sql-mode=value[,value[,value...]]
Set the SQL mode for MySQL. See section 5.2.2 The Server SQL Mode. This option was added in 3.23.41.
--temp-pool
This option causes most temporary files created by the server to use a small set of names, rather than a unique name for each new file. This works around a problem in the Linux kernel dealing with creating many new files with different names. With the old behavior, Linux seems to ``leak'' memory, because it's being allocated to the directory entry cache rather than to the disk cache.
--transaction-isolation=level
Sets the default transaction isolation level, which can be READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. See section 13.4.6 SET TRANSACTION Syntax.
--tmpdir=path, -t path
The path of the directory to use for creating temporary files. It might be useful if your default /tmp directory resides on a partition that is too small to hold temporary tables. Starting from MySQL 4.1, this option accepts several paths that are used in round-robin fashion. Paths should be separated by colon characters (`:') on Unix and semicolon characters (`;') on Windows, NetWare, and OS/2. If the MySQL server is acting as a replication slave, you should not set --tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication will fail.
--user={user_name | user_id}, -u {user_name | user_id}
Run the mysqld server as the user having the name user_name or the numeric user ID user_id. (``User'' in this context refers to a system login account, not a MySQL user listed in the grant tables.) This option is mandatory when starting mysqld as root. The server will change its user ID during its startup sequence, causing it to run as that particular user rather than as root. See section 5.4.1 General Security Guidelines. Starting from MySQL 3.23.56 and 4.0.12: To avoid a possible security hole where a user adds a --user=root option to some `my.cnf' file (thus causing the server to run as root), mysqld uses only the first --user option specified and produces a warning if there are multiple --user options. Options in `/etc/my.cnf' and `datadir/my.cnf' are processed before command-line options, so it is recommended that you put a --user option in `/etc/my.cnf' and specify a value other than root. The option in `/etc/my.cnf' will be found before any other --user options, which ensures that the server runs as a user other than root, and that a warning results if any other --user option is found.
--version, -V
Display version information and exit.

As of MySQL 4.0, you can assign a value to a server system variable by using an option of the form --var_name=value. For example, --key_buffer_size=32M sets the key_buffer_size variable to a value of 32MB.

Note that when setting a variable to a value, MySQL might automatically correct it to stay within a given range, or adjust the value to the closest allowable value if only certain values are allowed.

It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. However, this syntax is deprecated as of MySQL 4.0.

You can find a full description for all variables in section 5.2.3 Server System Variables. The section on tuning server parameters includes information on how to optimize them. See section 7.5.2 Tuning Server Parameters.

You can change the values of most system variables for a running server with the SET statement. See section 13.5.3 SET Syntax.

If you want to restrict the maximum value that a startup option can be set to with SET, you can define this by using the --maximum-var_name command-line option.

5.2.2 The Server SQL Mode

The MySQL server can operate in different SQL modes, and (as of MySQL 4.1) can apply these modes differentially for different clients. This allows an application to tailor server operation to its own requirements.

Modes define what SQL syntax MySQL should support and what kind of data validation checks it should perform. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.

You can set the default SQL mode by starting mysqld with the --sql-mode="modes" option. The value also can be empty (--sql-mode="") if you want to reset it.

Beginning with MySQL 4.1, you can also change the SQL mode after startup time by setting the sql_mode variable with a SET [SESSION|GLOBAL] sql_mode='modes' statement. Setting the GLOBAL variable requires the SUPER privilege and affects the operation of all clients that connect from that time on. Setting the SESSION variable affects only the current client. Any client can change its session sql_mode value.

modes is a list of different modes separated by comma (`,') characters. You can retrieve the current mode by issuing a SELECT @@sql_mode statement. The default value is empty (no modes set).

The most important sql_mode values are probably these:

ANSI
Change syntax and behavior to be more conformant to standard SQL. (New in MySQL 4.1.1)
STRICT_TRANS_TABLES
If a value could not be inserted as given into a transactional table, abort the statement. For a non-transactional table, abort the statement if the value occurs in a single-row statement or the first row of a multiple-row statement. More detail is given later in this section. (New in MySQL 5.0.2)
TRADITIONAL
Make MySQL behave like a ``traditional'' SQL database system. A simple description of this mode is ``give an error instead of a warning'' when inserting an incorrect value into a column. Note: The INSERT/UPDATE will abort as soon as the error is noticed. This may not be what you want if you are using a non-transactional storage engine, because data changes made prior to the error will not be rolled back, resulting in a ``partially-done'' update. (New in MySQL 5.0.2)

When this manual refers to ``strict mode,'' it means a mode where at least one of STRICT_TRANS_TABLES or STRICT_ALL_TABLES is enabled.

The following list describes all the supported modes:

ALLOW_INVALID_DATES
Don't do full checking of dates in strict mode. Check only that the month is in the range from 1 to 12 and the day is in the range from 1 to 31. This is very convenient for Web applications where you obtain year, month, and day in three different fields and you want to store exactly what the user inserted (without date validation). This mode applies to DATE and DATETIME columns. It does not apply TIMESTAMP columns, which always require a valid date. This mode is new in MySQL 5.0.2. Before 5.0.2, this was the default MySQL date-handling mode. As of 5.0.2, enabling strict mode causes the server to require that month and day values be legal, not just in the range from 1 to 12 and 1 to 31. For example, '2004-04-31' is legal with strict mode disabled, but illegal with strict mode enabled. To allow such dates in strict mode, enable ALLOW_INVALID_DATES as well.
ANSI_QUOTES
Treat `"' as an identifier quote character (like the ``' quote character) and not as a string quote character. You can still use ``' to quote identifers in ANSI mode. With ANSI_QUOTES enabled, you cannot use double quotes to quote a literal string, because it will be interpreted as an identifier. (New in MySQL 4.0.0)
ERROR_FOR_DIVISION_BY_ZERO
Produce an error in strict mode (otherwise a warning) when we encounter a division by zero (or MOD(X,0)) during an INSERT/ UPDATE. If this mode is not given, MySQL instead returns NULL for divisions by zero. If used with IGNORE, MySQL generates a warning for divisions by zero, but the result of the operation is NULL. (New in MySQL 5.0.2)
HIGH_NOT_PRECEDENCE
From MySQL 5.0.2 on, the NOT operator precedence is handled so that expressions such as NOT a BETWEEN b AND c are parsed as NOT (a BETWEEN b AND c). Before MySQL 5.0.2, the expression is parsed as (NOT a) BETWEEN b AND c. The old higher-precedence behavior can be obtained by enabling the HIGH_NOT_PRECEDENCE SQL mode. (New in MySQL 5.0.2)
mysql> SET sql_mode = '';
mysql> SELECT NOT 1 BETWEEN -5 AND 5;
        -> 0
mysql> SET sql_mode = 'broken_not';
mysql> SELECT NOT 1 BETWEEN -5 AND 5;
        -> 1
IGNORE_SPACE
Allow spaces between a function name and the `(' character. This forces all function names to be treated as reserved words. As a result, if you want to access any database, table, or column name that is a reserved word, you must quote it. For example, because there is a USER() function, the name of the user table in the mysql database and the User column in that table become reserved, so you must quote them:
SELECT "User" FROM mysql."user";
(New in MySQL 4.0.0)
NO_AUTO_CREATE_USER
Prevent GRANT from automatically creating new users if it would otherwise do so, unless a password also is specified. (New in MySQL 5.0.2)
NO_AUTO_VALUE_ON_ZERO
NO_AUTO_VALUE_ON_ZERO affects handling of AUTO_INCREMENT columns. Normally, you generate the next sequence number for the column by inserting either NULL or 0 into it. NO_AUTO_VALUE_ON_ZERO suppresses this behavior for 0 so that only NULL generates the next sequence number. (New in MySQL 4.1.1) This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (This is not a recommended practice, by the way.) For example, if you dump the table with mysqldump and then reload it, MySQL normally generates new sequence numbers when it encounters the 0 values, resulting in a table with different contents than the one that was dumped. Enabling NO_AUTO_VALUE_ON_ZERO before reloading the dump file solves this problem. As of MySQL 4.1.1, mysqldump automatically includes a statement in the dump output to enable NO_AUTO_VALUE_ON_ZERO.
NO_DIR_IN_CREATE
When creating a table, ignore all INDEX DIRECTORY and DATA DIRECTORY directives. This option is useful on slave replication servers. (New in MySQL 4.0.15)
NO_FIELD_OPTIONS
Don't print MySQL-specific column options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (New in MySQL 4.1.1)
NO_KEY_OPTIONS
Don't print MySQL-specific index options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (New in MySQL 4.1.1)
NO_TABLE_OPTIONS
Don't print MySQL-specific table options (such as ENGINE) in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (New in MySQL 4.1.1)
NO_UNSIGNED_SUBTRACTION
In subtraction operations, don't mark the result as UNSIGNED if one of the operands is unsigned. Note that this makes UNSIGNED BIGINT not 100% usable in all contexts. See section 12.7 Cast Functions and Operators. (New in MySQL 4.0.2)
NO_ZERO_DATE
Don't allow '0000-00-00' as a valid date. You can still insert zero dates with the IGNORE option. (New in MySQL 5.0.2)
NO_ZERO_IN_DATE
Don't accept dates where the month or day part is 0. If used with the IGNORE option, we insert a '0000-00-00' date for any such date. (New in MySQL 5.0.2)
ONLY_FULL_GROUP_BY
Don't allow queries that in the GROUP BY part refer to a not selected column. (New in MySQL 4.0.0)
PIPES_AS_CONCAT
Treat || as a string concatenation operator (same as CONCAT()) rather than as a synonym for OR. (New in MySQL 4.0.0)
REAL_AS_FLOAT
Treat REAL as a synonym for FLOAT rather than as a synonym for DOUBLE. (New in MySQL 4.0.0)
STRICT_ALL_TABLES
Enable strict mode for all storage engines. Invalid data values are rejected. Additional detail follows. (New in MySQL 5.0.2)
STRICT_TRANS_TABLES
Enable strict mode for transactional storage engines, and when possible for non-transactional storage engines. Additional detail follows. (New in MySQL 5.0.2)

Strict mode controls how MySQL handles values that are invalid or missing. A value can be invalid for several reasons. For example, it might have the wrong data type for the column, or it might be out of range. A value is missing when a new row to be inserted does not contain a value for a column that has no explicit DEFAULT clause in its definition.

For transactional tables, an error occurs for invalid or missing values in a statement when either of the STRICT_ALL_TABLES or STRICT_TRANS_TABLES modes are enabled. The statement is aborted and rolled back.

For non-transactional tables, the behavior is the same for either mode, if the bad value occurs in the first row to be inserted or updated. The statement is aborted and the table remains unchanged. If the statement inserts or modifies multiple rows and the bad value occurs in the second or later row, the result depends on which strict option is enabled:

Strict mode disallows invalid date values such as '2004-04-31'. It does not disallow dates with zero parts such as 2004-04-00' or ``zero'' dates. To disallow these as well, enable the NO_ZERO_IN_DATE and NO_ZERO_DATE SQL modes in addition to strict mode.

If you are not using strict mode (that is, neither STRICT_TRANS_TABLES nor STRICT_ALL_TABLES is enabled), MySQL inserts adjusted values for invalid or missing values and produces warnings. In strict mode, you can produce this behavior by using INSERT IGNORE or UPDATE IGNORE. See section 13.5.4.20 SHOW WARNINGS Syntax.

The following special modes are provided as shorthand for combinations of mode values from the preceding list. All are available as of MySQL 4.1.1, except TRADITIONAL (5.0.2).

The descriptions include all mode values that are available in the most recent version of MySQL. For older versions, a combination mode does not include individual mode values that are not available except in newer versions.

ANSI
Equivalent to REAL_AS_FLOAT, PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, ONLY_FULL_GROUP_BY. See section 1.5.3 Running MySQL in ANSI Mode.
DB2
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
MAXDB
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS, NO_AUTO_CREATE_USER.
MSSQL
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
MYSQL323
Equivalent to NO_FIELD_OPTIONS, HIGH_NOT_PRECEDENCE.
MYSQL40
Equivalent to NO_FIELD_OPTIONS, HIGH_NOT_PRECEDENCE.
ORACLE
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS, NO_AUTO_CREATE_USER.
POSTGRESQL
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
TRADITIONAL
Equivalent to STRICT_TRANS_TABLES, STRICT_ALL_TABLES, NO_ZERO_IN_DATE, NO_ZERO_DATE, ERROR_FOR_DIVISION_BY_ZERO, NO_AUTO_CREATE_USER.

5.2.3 Server System Variables

The server maintains many system variables that indicate how it is configured. All of them have default values. They can be set at server startup using options on the command line or in option files. Most of them can be set at runtime using the SET statement.

Beginning with MySQL 4.0.3, the mysqld server maintains two kinds of variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections.

When the server starts, it initializes all global variables to their default values. These defaults can be changed by options specified in option files or on the command line. After the server starts, those global variables that are dynamic can be changed by connecting to the server and issuing a SET GLOBAL var_name statement. To change a global variable, you must have the SUPER privilege.

The server also maintains a set of session variables for each client that connects. The client's session variables are initialized at connect time using the current values of the corresponding global variables. For those session variables that are dynamic, the client can change them by issuing a SET SESSION var_name statement. Setting a session variable requires no special privilege, but a client can change only its own session variables, not those of any other client.

A change to a global variable is visible to any client that accesses that global variable. However, it affects the corresponding session variable that is initialized from the global variable only for clients that connect after the change. It does not affect the session variable for any client that is already connected (not even that of the client that issues the SET GLOBAL statement).

When setting a variable using a startup option, variable values can be given with a suffix of K, M, or G to indicate kilobytes, megabytes, or gigabytes, respectively. For example, the following command starts the server with a key buffer size of 16 megabytes:

mysqld --key_buffer_size=16M

Before MySQL 4.0, use this syntax instead:

mysqld --set-variable=key_buffer_size=16M

The lettercase of suffix letters does not matter; 16M and 16m are equivalent.

At runtime, use the SET statement to set system variables. In this context, suffix letters cannot be used, but the value can take the form of an expression:

mysql> SET sort_buffer_size = 10 * 1024 * 1024;

To specify explicitly whether to set the global or session variable, use the GLOBAL or SESSION options:

mysql> SET GLOBAL sort_buffer_size = 10 * 1024 * 1024;
mysql> SET SESSION sort_buffer_size = 10 * 1024 * 1024;

Without either option, the statement sets the session variable.

The variables that can be set at runtime are listed in section 5.2.3.1 Dynamic System Variables.

If you want to restrict the maximum value to which a system variable can be set with the SET statement, you can specify this maximum by using an option of the form --maximum-var_name at server startup. For example, to prevent the value of query_cache_size from being increased to more than 32MB at runtime, use the option --maximum-query_cache_size=32M. This feature is available as of MySQL 4.0.2.

You can view system variables and their values by using the SHOW VARIABLES statement. See section 9.4 System Variables for more information.

mysql> SHOW VARIABLES;
+---------------------------------+------------------------------+
| Variable_name                   | Value                        |
+---------------------------------+------------------------------|
| back_log                        | 50                           |
| basedir                         | /usr/local/mysql             |
| bdb_cache_size                  | 8388572                      |
| bdb_home                        | /usr/local/mysql             |
| bdb_log_buffer_size             | 32768                        |
| bdb_logdir                      |                              |
| bdb_max_lock                    | 10000                        |
| bdb_shared_data                 | OFF                          |
| bdb_tmpdir                      | /tmp/                        |
| bdb_version                     | Sleepycat Software: ...      |
| binlog_cache_size               | 32768                        |
| bulk_insert_buffer_size         | 8388608                      |
| character_set                   | latin1                       |
| character_sets                  | latin1 big5 czech euc_kr     |
| concurrent_insert               | ON                           |
| connect_timeout                 | 5                            |
| convert_character_set           |                              |
| datadir                         | /usr/local/mysql/data/       |
| default_week_format             | 0                            |
| delay_key_write                 | ON                           |
| delayed_insert_limit            | 100                          |
| delayed_insert_timeout          | 300                          |
| delayed_queue_size              | 1000                         |
| flush                           | OFF                          |
| flush_time                      | 0                            |
| ft_boolean_syntax               | + -><()~*:""&|               |
| ft_max_word_len                 | 84                           |
| ft_min_word_len                 | 4                            |
| ft_query_expansion_limit        | 20                           |
| ft_stopword_file                | (built-in)                   |
| have_bdb                        | YES                          |
| have_innodb                     | YES                          |
| have_isam                       | YES                          |
| have_openssl                    | YES                          |
| have_query_cache                | YES                          |
| have_raid                       | NO                           |
| have_symlink                    | DISABLED                     |
| init_file                       |                              |
| innodb_additional_mem_pool_size | 1048576                      |
| innodb_buffer_pool_size         | 8388608                      |
| innodb_data_file_path           | ibdata1:10M:autoextend       |
| innodb_data_home_dir            |                              |
| innodb_fast_shutdown            | ON                           |
| innodb_file_io_threads          | 4                            |
| innodb_flush_log_at_trx_commit  | 1                            |
| innodb_flush_method             |                              |
| innodb_force_recovery           | 0                            |
| innodb_lock_wait_timeout        | 50                           |
| innodb_log_arch_dir             |                              |
| innodb_log_archive              | OFF                          |
| innodb_log_buffer_size          | 1048576                      |
| innodb_log_file_size            | 5242880                      |
| innodb_log_files_in_group       | 2                            |
| innodb_log_group_home_dir       | ./                           |
| innodb_mirrored_log_groups      | 1                            |
| innodb_thread_concurrency       | 8                            |
| interactive_timeout             | 28800                        |
| join_buffer_size                | 131072                       |
| key_buffer_size                 | 16773120                     |
| key_cache_age_threshold         | 300                          |
| key_cache_block_size            | 1024                         |
| key_cache_division_limit        | 100                          |
| language                        | /usr/local/mysql/share/...   |
| large_files_support             | ON                           |
| local_infile                    | ON                           |
| locked_in_memory                | OFF                          |
| log                             | OFF                          |
| log_bin                         | OFF                          |
| log_slave_updates               | OFF                          |
| log_slow_queries                | OFF                          |
| log_update                      | OFF                          |
| log_warnings                    | 1                            |
| long_query_time                 | 10                           |
| low_priority_updates            | OFF                          |
| lower_case_table_names          | 0                            |
| max_allowed_packet              | 1047552                      |
| max_binlog_cache_size           | 4294967295                   |
| max_binlog_size                 | 1073741824                   |
| max_connect_errors              | 10                           |
| max_connections                 | 100                          |
| max_delayed_threads             | 20                           |
| max_error_count                 | 64                           |
| max_heap_table_size             | 16777216                     |
| max_join_size                   | 4294967295                   |
| max_relay_log_size              | 0                            |
| max_sort_length                 | 1024                         |
| max_tmp_tables                  | 32                           |
| max_user_connections            | 0                            |
| max_write_lock_count            | 4294967295                   |
| myisam_max_extra_sort_file_size | 268435456                    |
| myisam_max_sort_file_size       | 2147483647                   |
| myisam_recover_options          | force                        |
| myisam_repair_threads           | 1                            |
| myisam_sort_buffer_size         | 8388608                      |
| net_buffer_length               | 16384                        |
| net_read_timeout                | 30                           |
| net_retry_count                 | 10                           |
| net_write_timeout               | 60                           |
| open_files_limit                | 1024                         |
| pid_file                        | /usr/local/mysql/name.pid    |
| port                            | 3306                         |
| protocol_version                | 10                           |
| query_cache_limit               | 1048576                      |
| query_cache_size                | 0                            |
| query_cache_type                | ON                           |
| read_buffer_size                | 131072                       |
| read_rnd_buffer_size            | 262144                       |
| rpl_recovery_rank               | 0                            |
| server_id                       | 0                            |
| skip_external_locking           | ON                           |
| skip_networking                 | OFF                          |
| skip_show_database              | OFF                          |
| slave_net_timeout               | 3600                         |
| slow_launch_time                | 2                            |
| socket                          | /tmp/mysql.sock              |
| sort_buffer_size                | 2097116                      |
| sql_mode                        |                              |
| table_cache                     | 64                           |
| table_type                      | MYISAM                       |
| thread_cache_size               | 3                            |
| thread_stack                    | 131072                       |
| timezone                        | EEST                         |
| tmp_table_size                  | 33554432                     |
| tmpdir                          | /tmp/:/mnt/hd2/tmp/          |
| tx_isolation                    | READ-COMMITTED               |
| version                         | 4.0.4-beta                   |
| wait_timeout                    | 28800                        |
+---------------------------------+------------------------------+

Most system variables are described here. Variables with no version indicated have been present since at least MySQL 3.22. InnoDB system variables are listed at section 15.5 InnoDB Startup Options.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

Information on tuning these variables can be found in section 7.5.2 Tuning Server Parameters.

ansi_mode
This is ON if mysqld was started with --ansi. See section 1.5.3 Running MySQL in ANSI Mode. This variable was added in MySQL 3.23.6 and removed in 3.23.41. See the description for sql_mode.
back_log
The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time. In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen() system call should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set back_log higher than your operating system limit will be ineffective.
basedir
The MySQL installation base directory. This variable can be set with the --basedir option.
bdb_cache_size
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you don't use BDB tables, you should start mysqld with --skip-bdb to not waste memory for this cache. This variable was added in MySQL 3.23.14.
bdb_home
The base directory for BDB tables. This should be assigned the same value as the datadir variable. This variable was added in MySQL 3.23.14.
bdb_log_buffer_size
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you don't use BDB tables, you should set this to 0 or start mysqld with --skip-bdb to not waste memory for this cache. This variable was added in MySQL 3.23.31.
bdb_logdir
The directory where the BDB storage engine writes its log files. This variable can be set with the --bdb-logdir option. This variable was added in MySQL 3.23.14.
bdb_max_lock
The maximum number of locks you can have active on a BDB table (10,000 by default). You should increase this if errors such as the following occur when you perform long transactions or when mysqld has to examine many rows to calculate a query:
bdb: Lock table is out of available locks
Got error 12 from ...
This variable was added in MySQL 3.23.29.
bdb_shared_data
This is ON if you are using --bdb-shared-data. This variable was added in MySQL 3.23.29.
bdb_tmpdir
The value of the --bdb-tmpdir option. This variable was added in MySQL 3.23.14.
bdb_version
See the description for version_bdb.
binlog_cache_size
The size of the cache to hold the SQL statements for the binary log during a transaction. A binary log cache is allocated for each client if the server supports any transactional storage engines and, starting from MySQL 4.1.2, if the server has binary log enabled (--log-bin option). If you often use big, multiple-statement transactions, you can increase this to get more performance. The Binlog_cache_use and Binlog_cache_disk_use status variables can be useful for tuning the size of this variable. This variable was added in MySQL 3.23.29. See section 5.9.4 The Binary Log.
bulk_insert_buffer_size
MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. Note: This cache is used only when adding data to a non-empty table. The default value is 8MB. This variable was added in MySQL 4.0.3. This variable previously was named myisam_bulk_insert_tree_size.
character_set
The default character set. This variable was added in MySQL 3.23.3, then removed in MySQL 4.1.1 and replaced by the various character_set_xxx variables.
character_set_client
The character set for statements that arrive from the client. This variable was added in MySQL 4.1.1.
character_set_connection
The character set used for literals that do not have a character set introducer and for number-to-string conversion. This variable was added in MySQL 4.1.1.
character_set_database
The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server. This variable was added in MySQL 4.1.1.
character_set_results
The character set used for returning query results to the client. This variable was added in MySQL 4.1.1.
character_set_server
The server default character set. This variable was added in MySQL 4.1.1.
character_set_system
The character set used by the server for storing identifiers. The value is always utf8. This variable was added in MySQL 4.1.1.
character_sets
The supported character sets. This variable was added in MySQL 3.23.15 and removed in MySQL 4.1.1. (Use SHOW CHARACTER SET for a list of character sets.)
character_sets_dir
The directory where character sets are installed. This variable was added in MySQL 4.1.2.
collation_connection
The collation of the connection character set. This variable was added in MySQL 4.1.1.
collation_database
The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server. This variable was added in MySQL 4.1.1.
collation_server
The server default collation. This variable was added in MySQL 4.1.1.
concurrent_insert
If ON (the default), MySQL allows INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle. You can turn this option off by starting mysqld with --safe or --skip-new. This variable was added in MySQL 3.23.7.
connect_timeout
The number of seconds the mysqld server waits for a connect packet before responding with Bad handshake.
convert_character_set
The current character set mapping that was set by SET CHARACTER SET. This variable was removed in MySQL 4.1.
datadir
The MySQL data directory. This variable can be set with the --datadir option.
default_week_format
The default mode value to use for the WEEK() function. This variable is available as of MySQL 4.0.14.
delay_key_write
This option applies only to MyISAM tables. It can have one of the following values to affect handling of the DELAY_KEY_WRITE table option that can be used in CREATE TABLE statements.
Option Description
OFF DELAYED_KEY_WRITE is ignored.
ON MySQL honors the DELAY_KEY_WRITE option for CREATE TABLE. This is the default value.
ALL All new opened tables are treated as if they were created with the DELAY_KEY_WRITE option enabled.
If DELAY_KEY_WRITE is enabled, this means that the key buffer for tables with this option are not flushed on every index update, but only when a table is closed. This will speed up writes on keys a lot, but if you use this feature, you should add automatic checking of all MyISAM tables by starting the server with the --myisam-recover option (for example, --myisam-recover=BACKUP,FORCE). See section 5.2.1 mysqld Command-Line Options and section 14.1.1 MyISAM Startup Options. Note that --external-locking doesn't offer any protection against index corruption for tables that use delayed key writes. This variable was added in MySQL 3.23.8.
delayed_insert_limit
After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it allows them to execute before continuing to insert delayed rows.
delayed_insert_timeout
How long an INSERT DELAYED handler thread should wait for INSERT statements before terminating.
delayed_queue_size
This is a per-table limit on the number of rows to queue when handling INSERT DELAYED statements. If the queue becomes full, any client that issues an INSERT DELAYED statement will wait until there is room in the queue again.
expire_logs_days
The number of days for automatic binary log removal. The default is 0, which means ``no automatic removal''. Possible removals happen at startup and at binary log rotation. This variable was added in MySQL 4.1.0.
flush
This is ON if you have started mysqld with the --flush option. This variable was added in MySQL 3.22.9.
flush_time
If this is set to a non-zero value, all tables will be closed every flush_time seconds to free up resources and sync unflushed data to disk. We recommend this option only on Windows 9x or Me, or on systems with minimal resources available. This variable was added in MySQL 3.22.18.
ft_boolean_syntax
The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. This variable was added in MySQL 4.0.1. See section 12.6.1 Boolean Full-Text Searches. The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:
ft_max_word_len
The maximum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0. Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
ft_min_word_len
The minimum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0. Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
ft_query_expansion_limit
The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION. This variable was added in MySQL 4.1.1.
ft_stopword_file
The file from which to read the list of stopwords for full-text searches. All the words from the file are used; comments are not honored. By default, a built-in list of stopwords is used (as defined in the `myisam/ft_static.c' file). Setting this variable to the empty string ('') disables stopword filtering. This variable was added in MySQL 4.0.10. Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
group_concat_max_len
The maximum allowed result length for the GROUP_CONCAT() function. This variable was added in MySQL 4.1.0.
have_archive
YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.3.
have_bdb
YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used. This variable was added in MySQL 3.23.30.
have_compress
Whether the zlib compression library is available to the server. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used. This variable was added in MySQL 4.1.1.
have_crypt
Whether the crypt() system call is available to the server. If not, the CRYPT() function cannot be used. This variable was added in MySQL 4.0.10.
have_csv
YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.4.
have_example_engine
YES if mysqld supports EXAMPLE tables, NO if not. This variable was added in MySQL 4.1.4.
have_geometry
Whether the server supports spatial data types. This variable was added in MySQL 4.1.3.
have_innodb
YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used. This variable was added in MySQL 3.23.37.
have_isam
YES if mysqld supports ISAM tables. DISABLED if --skip-isam is used. This variable was added in MySQL 3.23.30.
have_ndbcluster
YES if mysqld supports NDB Cluster tables. DISABLED if --skip-ndbcluster is used. This variable was added in MySQL 4.1.2.
have_openssl
YES if mysqld supports SSL (encryption) of the client/server protocol. This variable was added in MySQL 3.23.43.
have_query_cache
YES if mysqld supports the query cache. This variable was added in MySQL 4.0.2.
have_raid
YES if mysqld supports the RAID option. This variable was added in MySQL 3.23.30.
have_rtree_keys
Whether RTREE indexes are available. (These are used for spatial indexed in MyISAM tables.) This variable was added in MySQL 4.1.3.
have_symlink
Whether symbolic link support is enabled. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options. This variable was added in MySQL 4.0.0.
init_connect
A string to be executed by the server for each client that connects. The string consists of one or more SQL statements. To specify multiple statements, separate them by semicolon characters. For example, each client begins by default with autocommit mode enabled. There is no global server variable to specify that autocommit should be disabled by default, but init_connect can be used to achieve the same effect:
SET GLOBAL init_connect='SET AUTOCOMMIT=0';
This variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:
[mysqld]
init_connect='SET AUTOCOMMIT=0'
Note that the content of init_connect is not executed for users having the SUPER privilege; this is in case that content has been wrongly set (contains a wrong query, for example with a syntax error), thus making all connections fail. Not executing it for SUPER users enables those to open a connection and fix init_connect. This variable was added in MySQL 4.1.2.
init_file
The name of the file specified with the --init-file option when you start the server. This is a file containing SQL statements that you want the server to execute when it starts. Each statement must be on a single line and should not include comments. This variable was added in MySQL 3.23.2.
init_slave
This variable is similar to init_connect, but is a string to be executed by a slave server each time the SQL thread starts. The format of the string is the same as for the init_connect variable. This variable was added in MySQL 4.1.2.
innodb_xxx
The InnoDB system variables are listed at section 15.5 InnoDB Startup Options.
interactive_timeout
The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.
join_buffer_size
The size of the buffer that is used for full joins (joins that do not use indexes). Normally the best way to get fast joins is to add indexes. Increase the value of join_buffer_size to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.
key_buffer_size
Index blocks for MyISAM and ISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache. The maximum allowable setting for key_buffer_size is 4GB. The effective maximum size might be less, depending on your available physical RAM and per-process RAM limits imposed by your operating system or hardware platform. Increase the value to get better index handling (for all reads and multiple writes) to as much as you can afford. Using a value that is 25% of total memory on a machine that mainly runs MySQL is quite common. However, if you make the value too large (for example, more than 50% of your total memory) your system might start to page and become extremely slow. MySQL relies on the operating system to perform filesystem caching for data reads, so you must leave some room for the filesystem cache. For even more speed when writing many rows at the same time, use LOCK TABLES. See section 13.4.5 LOCK TABLES and UNLOCK TABLES Syntax. You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. See section 13.5.4 SHOW Syntax. The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the DELAY_KEY_WRITE table option. The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_unused status variable and the buffer block size. From MySQL 4.1.1 on, the buffer block size is available from the key_cache_block_size server variable. The fraction of the buffer in use is:
1 - ((Key_blocks_unused * key_cache_block_size) / key_buffer_size)
This value is an approximation because some space in the key buffer may be allocated internally for administrative structures. Before MySQL 4.1.1, key cache blocks are 1024 bytes, and before MySQL 4.1.2, Key_blocks_unused is unavailable. The Key_blocks_used variable can be used as follows to determine the fraction of the key buffer in use:
(Key_blocks_used * 1024) / key_buffer_size
However, Key_blocks_used indicates the maximum number of blocks that have ever been in use at once, so this formula does not necessary represent the current fraction of the buffer that is in use. See section 7.4.6 The MyISAM Key Cache.
key_cache_age_threshold
This value controls the demotion of buffers from the hot sub-chain of a key cache to the warm sub-chain. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. This variable was added in MySQL 4.1.1. See section 7.4.6 The MyISAM Key Cache.
key_cache_block_size
The size in bytes of blocks in the key cache. The default value is 1024. This variable was added in MySQL 4.1.1. See section 7.4.6 The MyISAM Key Cache.
key_cache_division_limit
The division point between the hot and warm sub-chains of the key cache buffer chain. The value is the percentage of the buffer chain to use for the warm sub-chain. Allowable values range from 1 to 100. The default value is 100. This variable was added in MySQL 4.1.1. See section 7.4.6 The MyISAM Key Cache.
language
The language used for error messages.
large_file_support
Whether mysqld was compiled with options for large file support. This variable was added in MySQL 3.23.28.
license
The type of license the server has. This variable was added in MySQL 4.0.19.
local_infile
Whether LOCAL is supported for LOAD DATA INFILE statements. This variable was added in MySQL 4.0.3.
locked_in_memory
Whether mysqld was locked in memory with --memlock. This variable was added in MySQL 3.23.25.
log
Whether logging of all queries to the general query log is enabled. See section 5.9.2 The General Query Log.
log_bin
Whether the binary log is enabled. This variable was added in MySQL 3.23.14. See section 5.9.4 The Binary Log.
log_error
The location of the error log. This variable was added in MySQL 4.0.10.
log_slave_updates
Whether updates received by a slave server from a master server should be logged to the slave's own binary log. Binary logging must be enabled on the slave for this to have any effect. This variable was added in MySQL 3.23.17. See section 6.8 Replication Startup Options.
log_slow_queries
Whether slow queries should be logged. ``Slow'' is determined by the value of the long_query_time variable. This variable was added in MySQL 4.0.2. See section 5.9.5 The Slow Query Log.
log_update
Whether the update log is enabled. This variable was added in MySQL 3.22.18. Note that the binary log is preferable to the update log, which is unavailable as of MySQL 5.0. See section 5.9.3 The Update Log.
log_warnings
Whether to produce additional warning messages. This variable was added in MySQL 4.0.3. It is enabled by default as of MySQL 4.0.19 and 4.1.2. As of MySQL 4.0.21 and 4.1.3, aborted connections are not logged to the error log unless the value is greater than 1.
long_query_time
If a query takes longer than this many seconds, the Slow_queries status variable is incremented. If you are using the --log-slow-queries option, the query is logged to the slow query log file. This value is measured in real time, not CPU time, so a query that is under the threshold on a lightly loaded system might be above the threshold on a heavily loaded one. See section 5.9.5 The Slow Query Log.
low_priority_updates
If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This variable previously was named sql_low_priority_updates. It was added in MySQL 3.22.5.
lower_case_file_system
This variable indicates whether the filesystem where the data directory is located has case insensitive filenames. ON means filenames are case insensitive, OFF means they are case sensitive. This variable was added in MySQL 4.0.19.
lower_case_table_names
If set to 1, table names are stored in lowercase on disk and table name comparisons are not case sensitive. This variable was added in MySQL 3.23.6. If set to 2 (new in 4.0.18), table names are stored as given but compared in lowercase. From MySQL 4.0.2, this option also applies to database names. From 4.1.1, it also applies to table aliases. See section 9.2.2 Identifier Case Sensitivity. You should not set this variable to 0 if you are running MySQL on a system that does not have case-sensitive filenames (such as Windows or Mac OS X). New in 4.0.18: If this variable is not set at startup and the filesystem on which the data directory is located does not have case-sensitive filenames, MySQL automatically sets lower_case_table_names to 2.
max_allowed_packet
The maximum size of one packet or any generated/intermediate string. The packet message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch big (possibly wrong) packets. You must increase this value if you are using big BLOB columns or long strings. It should be as big as the biggest BLOB you want to use. The protocol limit for max_allowed_packet is 16MB before MySQL 4.0 and 1GB thereafter.
max_binlog_cache_size
If a multiple-statement transaction requires more than this amount of memory, you will get the error Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage. This variable was added in MySQL 3.23.29.
max_binlog_size
If a write to the binary log exceeds the given value, rotate the binary logs. You cannot set this variable to more than 1GB or to less than 4096 bytes. (The minimum before MYSQL 4.0.14 is 1024 bytes.) The default value is 1GB. This variable was added in MySQL 3.23.33. Note if you are using transactions: A transaction is written in one chunk to the binary log, hence it is never split between several binary logs. Therefore, if you have big transactions, you might see binary logs bigger than max_binlog_size. If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well. max_relay_log_size was added in MySQL 4.0.14.
max_connect_errors
If there are more than this number of interrupted connections from a host, that host is blocked from further connections. You can unblock blocked hosts with the FLUSH HOSTS statement.
max_connections
The number of simultaneous client connections allowed. Increasing this value increases the number of file descriptors that mysqld requires. See section 7.4.8 How MySQL Opens and Closes Tables for comments on file descriptor limits. Also see section A.2.6 Too many connections.
max_delayed_threads
Don't start more than this number of threads to handle INSERT DELAYED statements. If you try to insert data into a new table after all INSERT DELAYED threads are in use, the row will be inserted as if the DELAYED attribute wasn't specified. If you set this to 0, MySQL never creates a thread to handle DELAYED rows; in effect, this disables DELAYED entirely. This variable was added in MySQL 3.23.0.
max_error_count
The maximum number of error, warning, and note messages to be stored for display by SHOW ERRORS or SHOW WARNINGS. This variable was added in MySQL 4.1.0.
max_heap_table_size
This variable sets the maximum size to which MEMORY (HEAP) tables are allowed to grow. The value of the variable is used to calculate MEMORY table MAX_ROWS values. Setting this variable has no effect on any existing MEMORY table, unless the table is re-created with a statement such as CREATE TABLE or TRUNCATE TABLE, or altered with ALTER TABLE. This variable was added in MySQL 3.23.0.
max_insert_delayed_threads
This variable is a synonym for max_delayed_threads. It was added in MySQL 4.0.19.
max_join_size
Don't allow SELECT statements that probably will need to examine more than max_join_size row combinations or are likely to do more than max_join_size disk seeks. By setting this value, you can catch SELECT statements where keys are not used properly and that would probably take a long time. Set it if your users tend to perform joins that lack a WHERE clause, that take a long time, or that return millions of rows. Setting this variable to a value other than DEFAULT resets the SQL_BIG_SELECTS value to 0. If you set the SQL_BIG_SELECTS value again, the max_join_size variable is ignored. If a query result already is in the query cache, no result size check is performed, because the result has already been computed and it does not burden the server to send it to the client. This variable previously was named sql_max_join_size.
max_length_for_sort_data
The cutoff on the size of index values that determines which filesort algorithm to use. See section 7.2.10 How MySQL Optimizes ORDER BY. This variable was added in MySQL 4.1.1
max_relay_log_size
If a write by a replication slave to its relay log exceeds the given value, rotate the relay log. This variable enables you to put different size constraints on relay logs and binary logs. However, setting the variable to 0 makes MySQL use max_binlog_size for both binary logs and relay logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. This variable was added in MySQL 4.0.14. See section 6.3 Replication Implementation Details.
max_seeks_for_key
Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer will assume that no more than this number of key seeks will be required when searching for matching rows in a table by scanning a key, regardless of the actual cardinality of the key (see section 13.5.4.11 SHOW INDEX Syntax). By setting this to a low value (100?), you can force MySQL to prefer keys instead of table scans. This variable was added in MySQL 4.0.14.
max_sort_length
The number of bytes to use when sorting BLOB or TEXT values. Only the first max_sort_length bytes of each value are used; the rest are ignored.
max_tmp_tables
The maximum number of temporary tables a client can keep open at the same time. (This option doesn't yet do anything.)
max_user_connections
The maximum number of simultaneous connections allowed to any given MySQL account. A value of 0 means ``no limit.'' This variable was added in MySQL 3.23.34.
max_write_lock_count
After this many write locks, allow some read locks to run in between. This variable was added in MySQL 3.23.7.
myisam_data_pointer_size
The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 8. The default value is 4. This variable was added in MySQL 4.1.2. See section A.2.11 The table is full.
myisam_max_extra_sort_file_size
If the temporary file used for fast MyISAM index creation would be larger than using the key cache by the amount specified here, prefer the key cache method. This is mainly used to force long character keys in large tables to use the slower key cache method to create the index. This variable was added in MySQL 3.23.37. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.
myisam_max_sort_file_size
The maximum size of the temporary file MySQL is allowed to use while re-creating a MyISAM index (during REPAIR TABLE, ALTER TABLE, or LOAD DATA INFILE). If the file size would be bigger than this value, the index will be created using the key cache instead, which is slower. This variable was added in MySQL 3.23.37. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.
myisam_recover_options
The value of the --myisam-recover option. This variable was added in MySQL 3.23.36.
myisam_repair_threads
If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process. The default value is 1. Note: Multi-threaded repair is still alpha quality code. This variable was added in MySQL 4.0.13.
myisam_sort_buffer_size
The buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE. This variable was added in MySQL 3.23.16.
named_pipe
On Windows, indicates whether the server supports connections over named pipes. This variable was added in MySQL 3.23.50.
net_buffer_length
The communication buffer is reset to this size between queries. This should not normally be changed, but if you have very little memory, you can set it to the expected length of SQL statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to max_allowed_packet bytes.
net_read_timeout
The number of seconds to wait for more data from a connection before aborting the read. When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout. This variable was added in MySQL 3.23.20.
net_retry_count
If a read on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads. This variable was added in MySQL 3.23.7.
net_write_timeout
The number of seconds to wait for a block to be written to a connection before aborting the write. See also net_read_timeout. This variable was added in MySQL 3.23.20.
new
This variable is used in MySQL 4.0 to turn on some 4.1 behaviors. This variable was added in MySQL 4.0.12.
old_passwords
Whether the server should use pre-4.1-style passwords for MySQL user accounts. This variable was added in MySQL 4.1.1.
open_files_limit
The number of files that the operating system allows mysqld to open. This is the real value allowed by the system and might be different from the value you gave mysqld as a startup option. The value is 0 on systems where MySQL can't change the number of open files. This variable was added in MySQL 3.23.20.
optimizer_prune_level
Controls the heuristics applied during query optimization to prune less-promising partial plans from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows retrieved by intermediate plans. This variable was added in MySQL 5.0.1.
optimizer_search_depth
The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value. If set to the maximum number of tables used in a query plus 2, the optimizer switches to the original algorithm used before MySQL 5.0.1 that performs an exhaustive search. This variable was added in MySQL 5.0.1.
pid_file
The pathname of the process ID (PID) file. This variable can be set with the --pid-file option. This variable was added in MySQL 3.23.23.
port
The port on which the server listens for TCP/IP connections. This variable can be set with the --port option.
preload_buffer_size
The size of the buffer that is allocated when preloading indexes. This variable was added in MySQL 4.1.1.
protocol_version
The version of the client/server protocol used by the MySQL server. This variable was added in MySQL 3.23.18.
query_alloc_block_size
The allocation size of memory blocks that are allocated for objects created during query parsing and execution. If you have problems with memory fragmentation, it might help to increase this a bit. This variable was added in MySQL 4.0.16.
query_cache_limit
Don't cache results that are bigger than this. The default value is 1MB. This variable was added in MySQL 4.0.1.
query_cache_min_res_unit
The minimum size for blocks allocated by the query cache. The default value is 4KB. Tuning information for this variable is given in section 5.11.3 Query Cache Configuration. This variable is present from MySQL 4.1.
query_cache_size
The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. Note that this amount of memory will be allocated even if query_cache_type is set to 0. This variable was added in MySQL 4.0.1.
query_cache_type
Set query cache type. Setting the GLOBAL value sets the type for all clients that connect thereafter. Individual clients can set the SESSION value to affect their own use of the query cache.
Option Description
0 or OFF Don't cache or retrieve results. Note that this will not deallocate the query cache buffer. To do that, you should set query_cache_size to 0.
1 or ON Cache all query results except for those that begin with SELECT SQL_NO_CACHE.
2 or DEMAND Cache results only for queries that begin with SELECT SQL_CACHE.
This variable was added in MySQL 4.0.3.
query_cache_wlock_invalidate
Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing queries for the table if the query results are present in the query cache. Setting this variable to 1 causes acquisition of a WRITE lock for a table to invalidate any queries in the query cache that refer to the table. This forces other clients that attempt to access the table to wait while the lock is in effect. This variable was added in MySQL 4.0.19.
query_prealloc_size
The size of the persistent buffer used for query parsing and execution. This buffer is not freed between queries. If you are running complex queries, a larger query_prealloc_size value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations. This variable was added in MySQL 4.0.16.
range_alloc_block_size
The size of blocks that are allocated when doing range optimization. This variable was added in MySQL 4.0.16.
read_buffer_size
Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you might want to increase this value. This variable was added in MySQL 4.0.3. Previously, it was named record_buffer.
read_only
When the variable is set to ON for a replication slave server, it causes the slave to allow no updates except from slave threads or from users with the SUPER privilege. This can be useful to ensure that a slave server accepts no updates from clients. This variable was added in MySQL 4.0.14.
relay_log_purge
Disables or enables automatic purging of relay logs as soon as they are not needed any more. The default value is 1 (enabled). This variable was added in MySQL 4.1.1.
read_rnd_buffer_size
When reading rows in sorted order after a sort, the rows are read through this buffer to avoid disk seeks. Setting the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries. This variable was added in MySQL 4.0.3. Previously, it was named record_rnd_buffer.
safe_show_database
Don't show databases for which the user has no database or table privileges. This can improve security if you're concerned about people being able to see what databases other users have. See also skip_show_database. This variable was removed in MySQL 4.0.5. Instead, use the SHOW DATABASES privilege to control access by MySQL accounts to database names.
secure_auth
If the MySQL server has been started with the --secure-auth option, it blocks connections from all accounts that have passwords stored in the old (pre-4.1) format. In that case, the value of this variable is ON, otherwise it is OFF. You should enable this option if you want to prevent all usage of passwords in old format (and hence insecure communication over the network). This variable was added in MySQL 4.1.1. Server startup will fail with an error if this option is enabled and the privilege tables are in pre-4.1 format. When used as a client-side option, the client refuses to connect to a server if the server requires a password in old format for the client account.
server_id
The value of the --server-id option. It is used for master and slave replication servers. This variable was added in MySQL 3.23.26.
shared_memory
Whether or not the server allows shared-memory connections. Currently, only Windows servers support this. This variable was added in MySQL 4.1.1.
shared_memory_base_name
Indicates whether or not the server allows shared-memory connections, and sets the identifier for the shared memory. This is useful when running multiple MYSQL instances on a single physical machine. Currently, only Windows servers support this. This variable was added in MySQL 4.1.0.
skip_external_locking
This is OFF if mysqld uses external locking. This variable was added in MySQL 4.0.3. Previously, it was named skip_locking.
skip_networking
This is ON if the server allows only local (non-TCP/IP) connections. On Unix, local connections use a Unix socket file. On Windows, local connections use a named pipe or shared memory. On NetWare, only TCP/IP connections are supported, so do not set this variable to ON. This variable was added in MySQL 3.22.23.
skip_show_database
This prevents people from using the SHOW DATABASES statement if they don't have the SHOW DATABASES privilege. This can improve security if you're concerned about people being able to see what databases other users have. See also safe_show_database. This variable was added in MySQL 3.23.4. As of MySQL 4.0.2, its effect also depends on the SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database.
slave_compressed_protocol
Whether to use compression of the slave/master protocol if both the slave and the master support it. This variable was added in MySQL 4.0.3.
slave_net_timeout
The number of seconds to wait for more data from a master/slave connection before aborting the read. This variable was added in MySQL 3.23.40.
slow_launch_time
If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable. This variable was added in MySQL 3.23.15.
socket
On Unix, this is the Unix socket file used for local client connections. On Windows, this is the name of the named pipe used for local client connections.
sort_buffer_size
Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster ORDER BY or GROUP BY operations. See section A.4.4 Where MySQL Stores Temporary Files.
sql_mode
The current server SQL mode. This variable was added in MySQL 3.23.41. It can be set dynamically as of MySQL 4.1.1. See section 5.2.2 The Server SQL Mode.
sql_slave_skip_counter
The number of events from the master that a slave server should skip. It was added in MySQL 3.23.33.
storage_engine
This variable is a synonym for table_type. It was added in MySQL 4.1.2.
sync_binlog
If positive, the MySQL server will synchronize its binary log to disk (fdatasync()) after every sync_binlog'th write to this binary log. Note that there is one write to the binary log per statement if in autocommit mode, and otherwise one write per transaction. The default value is 0 which does no sync'ing to disk. A value of 1 is the safest choice, because in case of crash you will lose at most one statement/transaction from the binary log; but it is also the slowest choice (unless the disk has a battery-backed cache, which makes sync'ing very fast). This variable was added in MySQL 4.1.3.
sync_frm
This was added as a command-line option in MySQL 4.0.18, and is also a settable global variable since MySQL 4.1.3. If set to 1, when a non-temporary table is created it will synchronize its `.frm' file to disk (fdatasync()); this is slower but safer in case of crash. Default is 1.
system_time_zone
The server system time zone. When the server begins executing, it inherits a time zone setting from the machine defaults, possibly modified by the environment of the account used for running the server or the startup script. The value is used to set system_time_zone. Typically the time zone is specified by the TZ environment variable. It also can be specified using the --timezone option of the mysqld_safe script. This variable was added in MySQL 4.1.3.
table_cache
The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See section 5.2.4 Server Status Variables. If the value of Opened_tables is large and you don't do FLUSH TABLES a lot (which just forces all tables to be closed and reopened), then you should increase the value of the table_cache variable. For more information about the table cache, see section 7.4.8 How MySQL Opens and Closes Tables.
table_type
The default table type (storage engine). To set the table type at server startup, use the --default-table-type option. This variable was added in MySQL 3.23.0. See section 5.2.1 mysqld Command-Line Options.
thread_cache_size
How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there aren't already thread_cache_size threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. (Normally this doesn't give a notable performance improvement if you have a good thread implementation.) By examining the difference between the Connections and Threads_created status variables (see section 5.2.4 Server Status Variables for details) you can see how efficient the thread cache is. This variable was added in MySQL 3.23.16.
thread_concurrency
On Solaris, mysqld calls thr_setconcurrency() with this value. This function allows applications to give the threads system a hint about the desired number of threads that should be run at the same time. This variable was added in MySQL 3.23.7.
thread_stack
The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. The default is large enough for normal operation. See section 7.1.4 The MySQL Benchmark Suite.
time_zone
The current time zone. The initial value of this is 'SYSTEM' (use the value of system_time_zone), but can be specified explicitly at server startup time with the --default-time-zone option. This variable was added in MySQL 4.1.3.
timezone
The time zone for the server. This is set from the TZ environment variable when mysqld is started. The time zone also can be set by giving a --timezone argument to mysqld_safe. This variable was added in MySQL 3.23.15. As of MySQL 4.1.3, it is obsolete and has been replaced by the system_time_zone variable. See section A.4.6 Time Zone Problems.
tmp_table_size
If an in-memory temporary table exceeds this size, MySQL automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size if you do many advanced GROUP BY queries and you have lots of memory.
tmpdir
The directory used for temporary files and temporary tables. Starting from MySQL 4.1, this variable can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (`:') on Unix and semicolon characters (`;') on Windows, NetWare, and OS/2. This feature can be used to spread the load between several physical disks. If the MySQL server is acting as a replication slave, you should not set tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication will fail. This variable was added in MySQL 3.22.4.
transaction_alloc_block_size
The allocation size of memory blocks that are allocated for storing queries that are part of a transaction to be stored in the binary log when doing a commit. This variable was added in MySQL 4.0.16.
transaction_prealloc_size
The size of the persistent buffer for transaction_alloc_blocks that is not freed between queries. By making this big enough to fit all queries in a common transaction, you can avoid a lot of malloc() calls. This variable was added in MySQL 4.0.16.
tx_isolation
The default transaction isolation level. This variable was added in MySQL 4.0.3.
updatable_views_with_limit
This variable controls whether updates can be made using a view that does not contain a primary key in the underlying table, if the update contains a LIMIT clause. (Such updates often are generated by GUI tools.) An update is an UPDATE or DELETE statement. Primary key here means a PRIMARY KEY, or a UNIQUE index in which no column can contain NULL. The variable can have two values: This variable was added in MySQL 5.0.2.
version
The version number for the server.
version_bdb
The BDB storage engine version. This variable was added in MySQL 3.23.31 with the name bdb_version and renamed to version_bdb in MySQL 4.1.1.
version_comment
The configure script has a --with-comment option that allows a comment to be specified when building MySQL. This variable contains the value of that comment. This variable was added in MySQL 4.0.17.
version_compile_machine
The type of machine MySQL was built on. This variable was added in MySQL 4.1.1.
version_compile_os
The type of operating system MySQL was built on. This variable was added in MySQL 4.0.19.
wait_timeout
The number of seconds the server waits for activity on a non-interactive connection before closing it. On thread startup, the session wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.

5.2.3.1 Dynamic System Variables

Beginning with MySQL 4.0.3, many server system variables are dynamic and can be set at runtime using SET GLOBAL or SET SESSION. You can also select their values using SELECT. See section 9.4 System Variables.

The following table shows the full list of all dynamic system variables. The last column indicates for each variable whether GLOBAL or SESSION (or both) apply.

Variable Name Value Type Type
autocommit boolean SESSION
big_tables boolean SESSION
binlog_cache_size numeric GLOBAL
bulk_insert_buffer_size numeric GLOBAL | SESSION
character_set_client string GLOBAL | SESSION
character_set_connection string GLOBAL | SESSION
character_set_results string GLOBAL | SESSION
character_set_server string GLOBAL | SESSION
collation_connection string GLOBAL | SESSION
collation_server string GLOBAL | SESSION
concurrent_insert boolean GLOBAL
connect_timeout numeric GLOBAL
convert_character_set string GLOBAL | SESSION
default_week_format numeric GLOBAL | SESSION
delay_key_write OFF | ON | ALL GLOBAL
delayed_insert_limit numeric GLOBAL
delayed_insert_timeout numeric GLOBAL
delayed_queue_size numeric GLOBAL
error_count numeric SESSION
expire_logs_days numeric GLOBAL
flush boolean GLOBAL
flush_time numeric GLOBAL
foreign_key_checks boolean SESSION
ft_boolean_syntax numeric GLOBAL
group_concat_max_len numeric GLOBAL | SESSION
identity numeric SESSION
innodb_autoextend_increment numeric GLOBAL
innodb_max_purge_lag numeric GLOBAL
innodb_table_locks boolean GLOBAL | SESSION
insert_id boolean SESSION
interactive_timeout numeric GLOBAL | SESSION
join_buffer_size numeric GLOBAL | SESSION
key_buffer_size numeric GLOBAL
last_insert_id numeric SESSION
local_infile boolean GLOBAL
log_warnings numeric GLOBAL
long_query_time numeric GLOBAL | SESSION
low_priority_updates boolean GLOBAL | SESSION
max_allowed_packet numeric GLOBAL | SESSION
max_binlog_cache_size numeric GLOBAL
max_binlog_size numeric GLOBAL
max_connect_errors numeric GLOBAL
max_connections numeric GLOBAL
max_delayed_threads numeric GLOBAL
max_error_count numeric GLOBAL | SESSION
max_heap_table_size numeric GLOBAL | SESSION
max_insert_delayed_threads numeric GLOBAL
max_join_size numeric GLOBAL | SESSION
max_relay_log_size numeric GLOBAL
max_seeks_for_key numeric GLOBAL | SESSION
max_sort_length numeric GLOBAL | SESSION
max_tmp_tables numeric GLOBAL | SESSION
max_user_connections numeric GLOBAL
max_write_lock_count numeric GLOBAL
myisam_data_pointer_size numeric GLOBAL
myisam_max_extra_sort_file_size numeric GLOBAL | SESSION
myisam_max_sort_file_size numeric GLOBAL | SESSION
myisam_repair_threads numeric GLOBAL | SESSION
myisam_sort_buffer_size numeric GLOBAL | SESSION
net_buffer_length numeric GLOBAL | SESSION
net_read_timeout numeric GLOBAL | SESSION
net_retry_count numeric GLOBAL | SESSION
net_write_timeout numeric GLOBAL | SESSION
old_passwords numeric GLOBAL | SESSION
optimizer_prune_level numeric GLOBAL | SESSION
optimizer_search_depth numeric GLOBAL | SESSION
preload_buffer_size numeric GLOBAL | SESSION
query_alloc_block_size numeric GLOBAL | SESSION
query_cache_limit numeric GLOBAL
query_cache_size numeric GLOBAL
query_cache_type enumeration GLOBAL | SESSION
query_cache_wlock_invalidate boolean GLOBAL | SESSION
query_prealloc_size numeric GLOBAL | SESSION
range_alloc_block_size numeric GLOBAL | SESSION
read_buffer_size numeric GLOBAL | SESSION
read_only numeric GLOBAL
read_rnd_buffer_size numeric GLOBAL | SESSION
rpl_recovery_rank numeric GLOBAL
safe_show_database boolean GLOBAL
secure_auth boolean GLOBAL
server_id numeric GLOBAL
slave_compressed_protocol boolean GLOBAL
slave_net_timeout numeric GLOBAL
slow_launch_time numeric GLOBAL
sort_buffer_size numeric GLOBAL | SESSION
sql_auto_is_null boolean SESSION
sql_big_selects boolean SESSION
sql_big_tables boolean SESSION
sql_buffer_result boolean SESSION
sql_log_bin boolean SESSION
sql_log_off boolean SESSION
sql_log_update boolean SESSION
sql_low_priority_updates boolean GLOBAL | SESSION
sql_max_join_size numeric GLOBAL | SESSION
sql_mode enumeration GLOBAL | SESSION
sql_quote_show_create boolean SESSION
sql_safe_updates boolean SESSION
sql_select_limit numeric SESSION
sql_slave_skip_counter numeric GLOBAL
updatable_views_with_limit enumeration GLOBAL | SESSION
sql_warnings boolean SESSION
sync_binlog numeric GLOBAL
sync_frm boolean GLOBAL
storage_engine enumeration GLOBAL | SESSION
table_cache numeric GLOBAL
table_type enumeration GLOBAL | SESSION
thread_cache_size numeric GLOBAL
time_zone string GLOBAL | SESSION
timestamp boolean SESSION
tmp_table_size enumeration GLOBAL | SESSION
transaction_alloc_block_size numeric GLOBAL | SESSION
transaction_prealloc_size numeric GLOBAL | SESSION
tx_isolation enumeration GLOBAL | SESSION
unique_checks boolean SESSION
wait_timeout numeric GLOBAL | SESSION
warning_count numeric SESSION

Variables that are marked as ``string'' take a string value. Variables that are marked as ``numeric'' take a numeric value. Variables that are marked as ``boolean'' can be set to 0, 1, ON or OFF. Variables that are marked as ``enumeration'' normally should be set to one of the available values for the variable, but can also be set to the number that corresponds to the desired enumeration value. For enumeration-valued system variables, the first enumeration value corresponds to 0. This differs from ENUM columns, for which the first enumeration value corresponds to 1.

5.2.4 Server Status Variables

The server maintains many status variables that provide information about its operations. You can view these variables and their values by using the SHOW STATUS statement:

mysql> SHOW STATUS;
+--------------------------+------------+
| Variable_name            | Value      |
+--------------------------+------------+
| Aborted_clients          | 0          |
| Aborted_connects         | 0          |
| Bytes_received           | 155372598  |
| Bytes_sent               | 1176560426 |
| Connections              | 30023      |
| Created_tmp_disk_tables  | 0          |
| Created_tmp_files        | 60         |
| Created_tmp_tables       | 8340       |
| Delayed_errors           | 0          |
| Delayed_insert_threads   | 0          |
| Delayed_writes           | 0          |
| Flush_commands           | 1          |
| Handler_delete           | 462604     |
| Handler_read_first       | 105881     |
| Handler_read_key         | 27820558   |
| Handler_read_next        | 390681754  |
| Handler_read_prev        | 6022500    |
| Handler_read_rnd         | 30546748   |
| Handler_read_rnd_next    | 246216530  |
| Handler_update           | 16945404   |
| Handler_write            | 60356676   |
| Key_blocks_used          | 14955      |
| Key_read_requests        | 96854827   |
| Key_reads                | 162040     |
| Key_write_requests       | 7589728    |
| Key_writes               | 3813196    |
| Max_used_connections     | 0          |
| Not_flushed_delayed_rows | 0          |
| Not_flushed_key_blocks   | 0          |
| Open_files               | 2          |
| Open_streams             | 0          |
| Open_tables              | 1          |
| Opened_tables            | 44600      |
| Qcache_free_blocks       | 36         |
| Qcache_free_memory       | 138488     |
| Qcache_hits              | 79570      |
| Qcache_inserts           | 27087      |
| Qcache_lowmem_prunes     | 3114       |
| Qcache_not_cached        | 22989      |
| Qcache_queries_in_cache  | 415        |
| Qcache_total_blocks      | 912        |
| Questions                | 2026873    |
| Select_full_join         | 0          |
| Select_full_range_join   | 0          |
| Select_range             | 99646      |
| Select_range_check       | 0          |
| Select_scan              | 30802      |
| Slave_open_temp_tables   | 0          |
| Slave_running            | OFF        |
| Slow_launch_threads      | 0          |
| Slow_queries             | 0          |
| Sort_merge_passes        | 30         |
| Sort_range               | 500        |
| Sort_rows                | 30296250   |
| Sort_scan                | 4650       |
| Table_locks_immediate    | 1920382    |
| Table_locks_waited       | 0          |
| Threads_cached           | 0          |
| Threads_connected        | 1          |
| Threads_created          | 30022      |
| Threads_running          | 1          |
| Uptime                   | 80380      |
+--------------------------+------------+

Many status variables are reset to 0 by the FLUSH STATUS statement.

The status variables have the following meanings. The Com_xxx statement counter variables were added beginning with MySQL 3.23.47. The Qcache_xxx query cache variables were added beginning with MySQL 4.0.1. Otherwise, variables with no version indicated have been present since at least MySQL 3.22.

Aborted_clients
The number of connections that were aborted because the client died without closing the connection properly. See section A.2.10 Communication Errors and Aborted Connections.
Aborted_connects
The number of tries to connect to the MySQL server that failed. See section A.2.10 Communication Errors and Aborted Connections.
Binlog_cache_disk_use
The number of transactions that used the temporary binary log cache but that exceeded the value of binlog_cache_size and used a temporary file to store statements from the transaction. This variable was added in MySQL 4.1.2.
Binlog_cache_use
The number of transactions that used the temporary binary log cache. This variable was added in MySQL 4.1.2.
Bytes_received
The number of bytes received from all clients. This variable was added in MySQL 3.23.7.
Bytes_sent
The number of bytes sent to all clients. This variable was added in MySQL 3.23.7.
Com_xxx
The number of times each xxx statement has been executed. There is one status variable for each type of statement. For example, Com_delete and Com_insert count DELETE and INSERT statements.
Connections
The number of connection attempts (successful or not) to the MySQL server.
Created_tmp_disk_tables
The number of temporary tables on disk created automatically by the server while executing statements. This variable was added in MySQL 3.23.24.
Created_tmp_files
How many temporary files mysqld has created. This variable was added in MySQL 3.23.28.
Created_tmp_tables
The number of in-memory temporary tables created automatically by the server while executing statements. If Created_tmp_disk_tables is big, you may want to increase the tmp_table_size value to cause temporary tables to be memory-based instead of disk-based.
Delayed_errors
The number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).
Delayed_insert_threads
The number of INSERT DELAYED handler threads in use.
Delayed_writes
The number of INSERT DELAYED rows written.
Flush_commands
The number of executed FLUSH statements.
Handler_commit
The number of internal COMMIT statements. This variable was added in MySQL 4.0.2.
Handler_discover
The MySQL server can ask the NDB Cluster storage engine if it knows about a table with a given name. This is called discovery. Handler_discover indicates the number of time tables have been discovered. This variable was added in MySQL 4.1.2.
Handler_delete
The number of times a row was deleted from a table.
Handler_read_first
The number of times the first entry was read from an index. If this is high, it suggests that the server is doing a lot of full index scans; for example, SELECT col1 FROM foo, assuming that col1 is indexed.
Handler_read_key
The number of requests to read a row based on a key. If this is high, it is a good indication that your queries and tables are properly indexed.
Handler_read_next
The number of requests to read the next row in key order. This will be incremented if you are querying an index column with a range constraint or if you are doing an index scan.
Handler_read_prev
The number of requests to read the previous row in key order. This read method is mainly used to optimize ORDER BY ... DESC. This variable was added in MySQL 3.23.6.
Handler_read_rnd
The number of requests to read a row based on a fixed position. This will be high if you are doing a lot of queries that require sorting of the result. You probably have a lot of queries that require MySQL to scan whole tables or you have joins that don't use keys properly.
Handler_read_rnd_next
The number of requests to read the next row in the data file. This will be high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have.
Handler_rollback
The number of internal ROLLBACK statements. This variable was added in MySQL 4.0.2.
Handler_update
The number of requests to update a row in a table.
Handler_write
The number of requests to insert a row in a table.
Innodb_buffer_pool_pages_data
The number of pages containing data (dirty or clean). Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_dirty
The number of pages currently dirty. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_flushed
The number of buffer pool pages that have been requested to be flushed. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_free
The number of free pages. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_latched
The number of latched pages in InnoDB buffer pool. These are pages currently being read or written or that can't be flushed or removed now for some other reason. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_misc
The number of pages busy because they have been allocated for administrative overhead such as row locks or the adaptive hash index. This value can also be calculated as Innodb_buffer_pool_pages_total - Innodb_buffer_pool_pages_free - Innodb_buffer_pool_pages_data. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_total
Total size of buffer pool, in pages. Added in MySQL 5.0.2.
Innodb_buffer_pool_read_ahead_rnd
The number of ``random'' read-aheads InnoDB initiated. This happens when a query is to scan a large portion of a table but in random order. Added in MySQL 5.0.2.
Innodb_buffer_pool_read_ahead_seq
The number of sequential read-aheads InnoDB initiated. This happens when InnoDB does a sequential full table scan. Added in MySQL 5.0.2.
Innodb_buffer_pool_read_requests
The number of logical read requests InnoDB has done. Added in MySQL 5.0.2.
Innodb_buffer_pool_reads
The number of logical reads that InnoDB could not satisfy from buffer pool and had to do a single-page read. Added in MySQL 5.0.2.
Innodb_buffer_pool_wait_free
Normally, writes to the InnoDB buffer pool happen in the background. However, if it's necessary to read or create a page and no clean pages are available, it's necessary to wait for pages to be flushed first. This counter counts instances of these waits. If the buffer pool size was set properly, this value should be small. Added in MySQL 5.0.2.
Innodb_buffer_pool_write_requests
The number writes done to the InnoDB buffer pool. Added in MySQL 5.0.2.
Innodb_data_fsyncs
The number of fsync() operations so far. Added in MySQL 5.0.2.
Innodb_data_pending_fsyncs
The current number of pending fsync() operations. Added in MySQL 5.0.2.
Innodb_data_pending_reads
The current number of pending reads. Added in MySQL 5.0.2.
Innodb_data_pending_writes
The current number of pending writes. Added in MySQL 5.0.2.
Innodb_data_read
The amount of data read so far, in bytes. Added in MySQL 5.0.2.
Innodb_data_reads
The total number of data reads. Added in MySQL 5.0.2.
Innodb_data_writes
The total number of data writes. Added in MySQL 5.0.2.
Innodb_data_written
The amount of data written so far, in bytes. Added in MySQL 5.0.2.
Innodb_dblwr_writes
Innodb_dblwr_pages_written
The number of doublewrite writes that have been performed and the number of pages that have been written for this purpose. Added in MySQL 5.0.2.
Innodb_log_waits
The number of waits we had because log buffer was too small and we had to wait for it to be flushed before continuing. Added in MySQL 5.0.2.
Innodb_log_write_requests
The number of log write requests. Added in MySQL 5.0.2.
Innodb_log_writes
The number of physical writes to the log file. Added in MySQL 5.0.2.
Innodb_os_log_fsyncs
The number of fsyncs writes done to the log file. Added in MySQL 5.0.2.
Innodb_os_log_pending_fsyncs
The number of pending log file fsyncs. Added in MySQL 5.0.2.
Innodb_os_log_pending_writes
Pending log file writes. Added in MySQL 5.0.2.
Innodb_os_log_written
The number of bytes written to the log file. Added in MySQL 5.0.2.
Innodb_page_size
The compiled-in InnoDB page size (default 16KB). Many values are counted in pages; the page size allows them to be easily converted to bytes. Added in MySQL 5.0.2.
Innodb_pages_created
The number of pages created. Added in MySQL 5.0.2.
Innodb_pages_read
The number of pages read. Added in MySQL 5.0.2.
Innodb_pages_written
The number of pages written. Added in MySQL 5.0.2.
Innodb_row_lock_current_waits
The number of row locks currently being waited for. Added in MySQL 5.0.3.
Innodb_row_lock_time
The total time spent in acquiring row locks, in milliseconds. Added in MySQL 5.0.3.
Innodb_row_lock_time_avg
The average time to acquire a row lock, in milliseconds. Added in MySQL 5.0.3.
Innodb_row_lock_time_max
The maximum time to acquire a row lock, in milliseconds. Added in MySQL 5.0.3.
Innodb_row_lock_waits
The number of times a row lock had to be waited for. Added in MySQL 5.0.3.
Innodb_rows_deleted
The number of rows deleted from InnoDB tables. Added in MySQL 5.0.2.
Innodb_rows_inserted
The number of rows inserted in InnoDB tables. Added in MySQL 5.0.2.
Innodb_rows_read
The number of rows read from InnoDB tables. Added in MySQL 5.0.2.
Innodb_rows_updated
The number of rows updated in InnoDB tables. Added in MySQL 5.0.2.
Key_blocks_not_flushed
The number of key blocks in the key cache that have changed but haven't yet been flushed to disk. This variable was added in MySQL 4.1.1. It used to be known as Not_flushed_key_blocks.
Key_blocks_unused
The number of unused blocks in the key cache. You can use this value to determine how much of the key cache is in use; see the discussion of key_buffer_size in section 5.2.3 Server System Variables. This variable was added in MySQL 4.1.2. section 5.2.3 Server System Variables.
Key_blocks_used
The number of used blocks in the key cache. This value is a high-water mark that indicates the maximum number of blocks that have ever been in use at one time.
Key_read_requests
The number of requests to read a key block from the cache.
Key_reads
The number of physical reads of a key block from disk. If Key_reads is big, then your key_buffer_size value is probably too small. The cache miss rate can be calculated as Key_reads/Key_read_requests.
Key_write_requests
The number of requests to write a key block to the cache.
Key_writes
The number of physical writes of a key block to disk.
Last_query_cost
The total cost of the last compiled query as computed by the query optimizer. Useful for comparing the cost of different query plans for the same query. The default value of -1 means that no query has been compiled yet. This variable was added in MySQL 5.0.1.
Max_used_connections
The maximum number of connections that have been in use simultaneously since the server started.
Not_flushed_delayed_rows
The number of rows waiting to be written in INSERT DELAY queues.
Not_flushed_key_blocks
The old name for Key_blocks_not_flushed before MySQL 4.1.1.
Open_files
The number of files that are open.
Open_streams
The number of streams that are open (used mainly for logging).
Open_tables
The number of tables that are open.
Opened_tables
The number of tables that have been opened. If Opened_tables is big, your table_cache value is probably too small.
Qcache_free_blocks
The number of free memory blocks in query cache.
Qcache_free_memory
The amount of free memory for query cache.
Qcache_hits
The number of cache hits.
Qcache_inserts
The number of queries added to the cache.
Qcache_lowmem_prunes
The number of queries that were deleted from the cache because of low memory.
Qcache_not_cached
The number of non-cached queries (not cachable, or due to query_cache_type).
Qcache_queries_in_cache
The number of queries registered in the cache.
Qcache_total_blocks
The total number of blocks in the query cache.
Questions
The number of queries that have been sent to the server.
Rpl_status
The status of failsafe replication (not yet implemented).
Select_full_join
The number of joins that do not use indexes. If this value is not 0, you should carefully check the indexes of your tables. This variable was added in MySQL 3.23.25.
Select_full_range_join
The number of joins that used a range search on a reference table. This variable was added in MySQL 3.23.25.
Select_range
The number of joins that used ranges on the first table. (It's normally not critical even if this is big.) This variable was added in MySQL 3.23.25.
Select_range_check
The number of joins without keys that check for key usage after each row. (If this is not 0, you should carefully check the indexes of your tables.) This variable was added in MySQL 3.23.25.
Select_scan
The number of joins that did a full scan of the first table. This variable was added in MySQL 3.23.25.
Slave_open_temp_tables
The number of temporary tables currently open by the slave SQL thread. This variable was added in MySQL 3.23.29.
Slave_running
This is ON if this server is a slave that is connected to a master. This variable was added in MySQL 3.23.16.
Slow_launch_threads
The number of threads that have taken more than slow_launch_time seconds to create. This variable was added in MySQL 3.23.15.
Slow_queries
The number of queries that have taken more than long_query_time seconds. See section 5.9.5 The Slow Query Log.
Sort_merge_passes
The number of merge passes the sort algorithm has had to do. If this value is large, you should consider increasing the value of the sort_buffer_size system variable. This variable was added in MySQL 3.23.28.
Sort_range
The number of sorts that were done with ranges. This variable was added in MySQL 3.23.25.
Sort_rows
The number of sorted rows. This variable was added in MySQL 3.23.25.
Sort_scan
The number of sorts that were done by scanning the table. This variable was added in MySQL 3.23.25.
Ssl_xxx
Variables used for SSL connections. These variables were added in MySQL 4.0.0.
Table_locks_immediate
The number of times that a table lock was acquired immediately. This variable was added as of MySQL 3.23.33.
Table_locks_waited
The number of times that a table lock could not be acquired immediately and a wait was needed. If this is high, and you have performance problems, you should first optimize your queries, and then either split your table or tables or use replication. This variable was added as of MySQL 3.23.33.
Threads_cached
The number of threads in the thread cache. This variable was added in MySQL 3.23.17.
Threads_connected
The number of currently open connections.
Threads_created
The number of threads created to handle connections. If Threads_created is big, you may want to increase the thread_cache_size value. The cache hit rate can be calculated as Threads_created/Connections. This variable was added in MySQL 3.23.31.
Threads_running
The number of threads that are not sleeping.
Uptime
The number of seconds the server has been up.

5.3 The MySQL Server Shutdown Process

The server shutdown process can be summarized like this:

  1. The shutdown process is initiated
  2. The server creates a shutdown thread if necessary
  3. The server stops accepting new connections
  4. The server terminates current activity
  5. Storage engines are shut down or closed
  6. The server exits

A more detailed description of the process follows:

  1. The shutdown process is initiated. Server shutdown can be initiated several ways. For example, a user with the SHUTDOWN privilege can execute a mysqladmin shutdown command. mysqladmin can be used on any platform supported by MySQL. Other operating sytem-specific shutdown initiation methods are possible as well: The server shuts down on Unix when it receives a SIGTERM signal. A server running as a service on Windows shuts down when the services manager tells it to.
  2. The server creates a shutdown thread if necessary. Depending on how shutdown was initiated, the server might create a thread to handle the shutdown process. If shutdown was requested by a client, a shutdown thread is created. If shutdown is the result of receiving a SIGTERM signal, the signal thread might handle shutdown itself, or it might create a separate thread to do so. If the server tries to create a shutdown thread and cannot (for example, if memory is exhausted), it issues a diagnostic message that will appear in the error log:
    Error: Can't create thread to kill server
    
  3. The server stops accepting new connections. To prevent new activity from being initiated during shutdown, the server stops accepting new client connections. It does this by closing the network connections to which it normally listens for connections: the TCP/IP port, the Unix socket file, the Windows named pipe, and shared memory on Windows.
  4. The server terminates current activity. For each thread that is associated with a client connection, the connection to the client is broken and the thread is marked as killed. Threads die when they notice that they are so marked. Threads for idle connections die quickly. Threads that currently are processing queries check their state periodically and take longer to die. For additional information about thread termination, see section 13.5.5.3 KILL Syntax, in particular for the instructions about killed REPAIR TABLE or OPTIMIZE TABLE operations on MyISAM tables. For threads that have an open transaction, the tranaction is rolled back. Note that if a thread is updating a non-transactional table, an operation such as a multiple-row UPDATE or INSERT may leave the table partially updated, because the operation can terminate before completion. If the server is a master replication server, threads associated with currently connected slaves are treated like other client threads. That is, each one is marked as killed and exits when it next checks its state. If the server is a slave replication server, the I/O and SQL threads, if active, are stopped before client threads are marked as killed. The SQL thread is allowed to finish its current statement (to avoid causing replication problems) then stops. If the SQL thread was in the middle of a transaction at this point, the transaction is rolled back.
  5. Storage engines are shut down or closed. At this stage, the table cache is flushed and all open tables are closed. Each storage engine performs any actions necessary for tables that it manages. For example, MyISAM flushes any pending index writes for a table. InnoDB flushes its buffer pool to disk, writes the current LSN to the tablespace, and terminates its own internal threads.
  6. The server exits.

5.4 General Security Issues

This section describes some general security issues to be aware of and what you can do to make your MySQL installation more secure against attack or misuse. For information specifically about the access control system that MySQL uses for setting up user accounts and checking database access, see section 5.5 The MySQL Access Privilege System.

5.4.1 General Security Guidelines

Anyone using MySQL on a computer connected to the Internet should read this section to avoid the most common security mistakes.

In discussing security, we emphasize the necessity of fully protecting the entire server host (not just the MySQL server) against all types of applicable attacks: eavesdropping, altering, playback, and denial of service. We do not cover all aspects of availability and fault tolerance here.

MySQL uses security based on Access Control Lists (ACLs) for all connections, queries, and other operations that users can attempt to perform. There is also some support for SSL-encrypted connections between MySQL clients and servers. Many of the concepts discussed here are not specific to MySQL at all; the same general ideas apply to almost all applications.

When running MySQL, follow these guidelines whenever possible:

5.4.2 Making MySQL Secure Against Attackers

When you connect to a MySQL server, you should use a password. The password is not transmitted in clear text over the connection. Password handling during the client connection sequence was upgraded in MySQL 4.1.1 to be very secure. If you are using an older version of MySQL, or are still using pre-4.1.1-style passwords, the encryption algorithm is less strong and with some effort a clever attacker who can sniff the traffic between the client and the server can crack the password. (See section 5.5.9 Password Hashing in MySQL 4.1 for a discussion of the different password handling methods.) If the connection between the client and the server goes through an untrusted network, you should use an SSH tunnel to encrypt the communication.

All other information is transferred as text that can be read by anyone who is able to watch the connection. If you are concerned about this, you can use the compressed protocol (in MySQL 3.22 and above) to make traffic much more difficult to decipher. To make the connection even more secure, you should use SSH to get an encrypted TCP/IP connection between a MySQL server and a MySQL client. You can find an Open Source SSH client at http://www.openssh.org/, and a commercial SSH client at http://www.ssh.com/.

If you are using MySQL 4.0 or newer, you can also use internal OpenSSL support. See section 5.6.7 Using Secure Connections.

To make a MySQL system secure, you should strongly consider the following suggestions:

5.4.3 Startup Options for mysqld Concerning Security

The following mysqld options affect security:

--local-infile[={0|1}]
If you start the server with --local-infile=0, clients cannot use LOCAL in LOAD DATA statements. See section 5.4.4 Security Issues with LOAD DATA LOCAL.
--safe-show-database
With this option, the SHOW DATABASES statement displays the names of only those databases for which the user has some kind of privilege. As of MySQL 4.0.2, this option is deprecated and doesn't do anything (it is enabled by default), because there is now a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis. See section 13.5.1.2 GRANT and REVOKE Syntax.
--safe-user-create
If this is enabled, a user cannot create new users with the GRANT statement unless the user has the INSERT privilege for the mysql.user table. If you want a user to have the ability to create new users with those privileges that the user has right to grant, you should grant the user the following privilege:
mysql> GRANT INSERT(user) ON mysql.user TO 'user_name'@'host_name';
This will ensure that the user can't change any privilege columns directly, but has to use the GRANT statement to give privileges to other users.
--secure-auth
Disallow authentication for accounts that have old (pre-4.1) passwords. This option is available as of MySQL 4.1.1.
--skip-grant-tables
This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing a mysqladmin flush-privileges or mysqladmin reload command, or by issuing a FLUSH PRIVILEGES statement.)
--skip-name-resolve
Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost.
--skip-networking
Don't allow TCP/IP connections over the network. All connections to mysqld must be made via Unix socket files. This option is unsuitable when using a MySQL version prior to 3.23.27 with the MIT-pthreads package, because Unix socket files were not supported by MIT-pthreads at that time.
--skip-show-database
With this option, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. Without this option, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database.

5.4.4 Security Issues with LOAD DATA LOCAL

The LOAD DATA statement can load a file that is located on the server host, or it can load a file that is located on the client host when the LOCAL keyword is specified.

There are two potential security issues with supporting the LOCAL version of LOAD DATA statements:

To deal with these problems, we changed how LOAD DATA LOCAL is handled as of MySQL 3.23.49 and MySQL 4.0.2 (4.0.13 on Windows):

5.5 The MySQL Access Privilege System

MySQL has an advanced but non-standard security/privilege system. This section describes how it works.

5.5.1 What the Privilege System Does

The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as SELECT, INSERT, UPDATE, and DELETE.

Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.

5.5.2 How the Privilege System Works

The MySQL privilege system ensures that all users may perform only the operations allowed to them. As a user, when you connect to a MySQL server, your identity is determined by the host from which you connect and the username you specify. The system grants privileges according to your identity and what you want to do.

MySQL considers both your hostname and username in identifying you because there is little reason to assume that a given username belongs to the same person everywhere on the Internet. For example, the user joe who connects from office.com need not be the same person as the user joe who connects from elsewhere.com. MySQL handles this by allowing you to distinguish users on different hosts that happen to have the same name: You can grant joe one set of privileges for connections from office.com, and a different set of privileges for connections from elsewhere.com.

MySQL access control involves two stages:

If your privileges are changed (either by yourself or someone else) while you are connected, those changes will not necessarily take effect immediately for the next statement you issue. See section 5.5.7 When Privilege Changes Take Effect for details.

The server stores privilege information in the grant tables of the mysql database (that is, in the database named mysql). The MySQL server reads the contents of these tables into memory when it starts and re-reads them under the circumstances indicated in section 5.5.7 When Privilege Changes Take Effect. Access-control decisions are based on the in-memory copies of the grant tables.

Normally, you manipulate the contents of the grant tables indirectly by using the GRANT and REVOKE statements to set up accounts and control the privileges available to each one. See section 13.5.1.2 GRANT and REVOKE Syntax. The discussion here describes the underlying structure of the grant tables and how the server uses their contents when interacting with clients.

The server uses the user, db, and host tables in the mysql database at both stages of access control. The columns in these grant tables are shown here:

Table Name user db host
Scope columns Host Host Host
User Db Db
Password User
Privilege columns Select_priv Select_priv Select_priv
Insert_priv Insert_priv Insert_priv
Update_priv Update_priv Update_priv
Delete_priv Delete_priv Delete_priv
Index_priv Index_priv Index_priv
Alter_priv Alter_priv Alter_priv
Create_priv Create_priv Create_priv
Drop_priv Drop_priv Drop_priv
Grant_priv Grant_priv Grant_priv
Create_view_priv Create_view_priv Create_view_priv
Show_view_priv Show_view_priv Show_view_priv
References_priv References_priv References_priv
Reload_priv
Shutdown_priv
Process_priv
File_priv
Show_db_priv
Super_priv
Create_tmp_table_priv Create_tmp_table_priv Create_tmp_table_priv
Lock_tables_priv Lock_tables_priv Lock_tables_priv
Execute_priv
Repl_slave_priv
Repl_client_priv
ssl_type
ssl_cipher
x509_issuer
x509_subject
max_questions
max_updates
max_connections

The ssl_type, ssl_cipher, x509_issuer, and x509_subject columns were added in MySQL 4.0.0.

The Create_tmp_table_priv, Execute_priv, Lock_tables_priv, Repl_client_priv, Repl_slave_priv, Show_db_priv, Super_priv, max_questions, max_updates, and max_connections columns were added in MySQL 4.0.2.

The Create_view_priv and Show_view_priv columns were added in MySQL 5.0.1.

During the second stage of access control (request verification), the server may, if the request involves tables, additionally consult the tables_priv and columns_priv tables that provide finer control at the table and column levels. The columns in these tables are shown here:

Table Name tables_priv columns_priv
Scope columns Host Host
Db Db
User User
Table_name Table_name
Column_name
Privilege columns Table_priv Column_priv
Column_priv
Other columns Timestamp Timestamp
Grantor

The Timestamp and Grantor columns currently are unused and are discussed no further here.

Each grant table contains scope columns and privilege columns:

Scope columns contain strings. They are declared as shown here; the default value for each is the empty string:

Column Name Type
Host CHAR(60)
User CHAR(16)
Password CHAR(16)
Db CHAR(64)
Table_name CHAR(60)
Column_name CHAR(60)

Before MySQL 3.23, the Db column is CHAR(32) in some tables and CHAR(60) in others.

For access-checking purposes, comparisons of Host values are case-insensitive. User, Password, Db, and Table_name values are case sensitive. Column_name values are case insensitive in MySQL 3.22.12 or later.

In the user, db, and host tables, each privilege is listed in a separate column that is declared as ENUM('N','Y') DEFAULT 'N'. In other words, each privilege can be disabled or enabled, with the default being disabled.

In the tables_priv and columns_priv tables, the privilege columns are declared as SET columns. Values in these columns can contain any combination of the privileges controlled by the table:

Table Name Column Name Possible Set Elements
tables_priv Table_priv 'Select', 'Insert', 'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'
tables_priv Column_priv 'Select', 'Insert', 'Update', 'References'
columns_priv Column_priv 'Select', 'Insert', 'Update', 'References'

Briefly, the server uses the grant tables as follows:

Administrative privileges (such as RELOAD or SHUTDOWN) are specified only in the user table. This is because administrative operations are operations on the server itself and are not database-specific, so there is no reason to list these privileges in the other grant tables. In fact, to determine whether you can perform an administrative operation, the server need consult only the user table.

The FILE privilege also is specified only in the user table. It is not an administrative privilege as such, but your ability to read or write files on the server host is independent of the database you are accessing.

The mysqld server reads the contents of the grant tables into memory when it starts. You can tell it to re-read the tables by issuing a FLUSH PRIVILEGES statement or executing a mysqladmin flush-privileges or mysqladmin reload command. Changes to the grant tables take effect as indicated in section 5.5.7 When Privilege Changes Take Effect.

When you modify the contents of the grant tables, it is a good idea to make sure that your changes set up privileges the way you want. One way to check the privileges for a given account is to use the SHOW GRANTS statement. For example, to determine the privileges that are granted to an account with Host and User values of pc84.example.com and bob, issue this statement:

mysql> SHOW GRANTS FOR 'bob'@'pc84.example.com';

A useful diagnostic tool is the mysqlaccess script, which Yves Carlier has provided for the MySQL distribution. Invoke mysqlaccess with the --help option to find out how it works. Note that mysqlaccess checks access using only the user, db, and host tables. It does not check table or column privileges specified in the tables_priv or columns_priv tables.

For additional help in diagnosing privilege-related problems, see section 5.5.8 Causes of Access denied Errors. For general advice on security issues, see section 5.4 General Security Issues.

5.5.3 Privileges Provided by MySQL

Information about account privileges is stored in the user, db, host, tables_priv, and columns_priv tables in the mysql database. The MySQL server reads the contents of these tables into memory when it starts and re-reads them under the circumstances indicated in section 5.5.7 When Privilege Changes Take Effect. Access-control decisions are based on the in-memory copies of the grant tables.

The names used in this manual to refer to the privileges provided by MySQL are shown in the following table, along with the table column name associated with each privilege in the grant tables and the context in which the privilege applies. Further information about the meaning of each privilege may be found at section 13.5.1.2 GRANT and REVOKE Syntax.

Privilege Column Context
ALTER Alter_priv tables
DELETE Delete_priv tables
INDEX Index_priv tables
INSERT Insert_priv tables
SELECT Select_priv tables
UPDATE Update_priv tables
CREATE Create_priv databases, tables, or indexes
DROP Drop_priv databases or tables
GRANT Grant_priv databases or tables
REFERENCES References_priv databases or tables
CREATE TEMPORARY TABLES Create_tmp_table_priv server administration
EXECUTE Execute_priv server administration
FILE File_priv file access on server host
LOCK TABLES Lock_tables_priv server administration
PROCESS Process_priv server administration
RELOAD Reload_priv server administration
REPLICATION CLIENT Repl_client_priv server administration
REPLICATION SLAVE Repl_slave_priv server administration
SHOW DATABASES Show_db_priv server administration
SHUTDOWN Shutdown_priv server administration
SUPER Super_priv server administration

The CREATE TEMPORARY TABLES, EXECUTE, LOCK TABLES, REPLICATION CLIENT, REPLICATION SLAVE, SHOW DATABASES, and SUPER privileges were added in MySQL 4.0.2.

The EXECUTE and REFERENCES privileges currently are unused.

The SELECT, INSERT, UPDATE, and DELETE privileges allow you to perform operations on rows in existing tables in a database.

SELECT statements require the SELECT privilege only if they actually retrieve rows from a table. Some SELECT statements do not access tables and can be executed without permission for any database. For example, you can use the mysql client as a simple calculator to evaluate expressions that make no reference to tables:

mysql> SELECT 1+1;
mysql> SELECT PI()*2;

The CREATE and DROP privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables. If you grant the DROP privilege for the mysql database to a user, that user can drop the database in which the MySQL access privileges are stored!

The INDEX privilege allows you to create or drop (remove) indexes. INDEX applies to existing tables. If you have the CREATE privilege for a table, you can include index definitions in the CREATE TABLE statement.

The ALTER privilege allows you to use ALTER TABLE to change the structure of or rename tables.

The GRANT privilege allows you to give to other users those privileges that you yourself possess.

The FILE privilege gives you permission to read and write files on the server host using the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements. A user who has the FILE privilege can read any file on the server host that is either world-readable or readable by the MySQL server. (This implies the user can read any file in any database directory, because the server can access any of those files.) The FILE privilege also allows the user to create new files in any directory where the MySQL server has write access. Existing files cannot be overwritten.

The remaining privileges are used for administrative operations. Many of them can be performed by using the mysqladmin program or by issuing SQL statements. The following table shows which mysqladmin commands each administrative privilege allows you to execute:

Privilege Commands Permitted to Privilege Holders
RELOAD flush-hosts, flush-logs, flush-privileges, flush-status, flush-tables, flush-threads, refresh, reload
SHUTDOWN shutdown
PROCESS processlist
SUPER kill

The reload command tells the server to re-read the grant tables into memory. flush-privileges is a synonym for reload. The refresh command closes and reopens the log files and flushes all tables. The other flush-xxx commands perform functions similar to refresh, but are more specific and may be preferable in some instances. For example, if you want to flush just the log files, flush-logs is a better choice than refresh.

The shutdown command shuts down the server. This command can be issued only from mysqladmin. There is no corresponding SQL statement.

The processlist command displays information about the threads executing within the server (that is, about the statements being executed by clients associated with other accounts). The kill command terminates server threads. You can always display or kill your own threads, but you need the PROCESS privilege to display threads initiated by other users and the SUPER privilege to kill them. See section 13.5.5.3 KILL Syntax. Prior to MySQL 4.0.2 when SUPER was introduced, the PROCESS privilege controls the ability to both see and terminate threads for other clients.

The CREATE TEMPORARY TABLES privilege allows the use of the keyword TEMPORARY in CREATE TABLE statements.

The LOCK TABLES privilege allows the use of explicit LOCK TABLES statements to lock tables for which you have the SELECT privilege. This includes the use of write locks, which prevents anyone else from reading the locked table.

The REPLICATION CLIENT privilege allows the use of SHOW MASTER STATUS and SHOW SLAVE STATUS.

The REPLICATION SLAVE privilege should be granted to accounts that are used by slave servers when they connect to the current server as their master. Without this privilege, the slave cannot request updates that have been made to databases on the master server.

The SHOW DATABASES privilege allows the account to see database names by issuing the SHOW DATABASE statement. Accounts that do not have this privilege see only databases for which they have some privileges, and cannot use the statement at all if the server was started with the --skip-show-database option.

It is a good idea in general to grant privileges to only those accounts that need them, but you should exercise particular caution in granting administrative privileges:

There are some things that you cannot do with the MySQL privilege system:

5.5.4 Connecting to the MySQL Server

MySQL client programs generally expect you to specify connection parameters when you want to access a MySQL server:

For example, the mysql client can be started as follows from a command-line prompt (indicated here by shell>):

shell> mysql -h host_name -u user_name -pyour_pass

Alternate forms of the -h, -u, and -p options are --host=host_name, --user=user_name, and --password=your_pass. Note that there is no space between -p or --password= and the password following it.

If you use a -p or --password option but do not specify the password value, the client program will prompt you to enter the password. The password is not displayed as you enter it. This is more secure than giving the password on the command line. Any user on your system may be able to see a password specified on the command line by executing a command such as ps auxww. See section 5.6.6 Keeping Your Password Secure.

MySQL client programs use default values for any connection parameter option that you do not specify:

Thus, for a Unix user with a login name of joe, all of the following commands are equivalent:

shell> mysql -h localhost -u joe
shell> mysql -h localhost
shell> mysql -u joe
shell> mysql

Other MySQL clients behave similarly.

You can specify different default values to be used when you make a connection so that you need not enter them on the command line each time you invoke a client program. This can be done in a couple of ways:

5.5.5 Access Control, Stage 1: Connection Verification

When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, then enters Stage 2 and waits for requests.

Your identity is based on two pieces of information:

Identity checking is performed using the three user table scope columns (Host, User, and Password). The server accepts the connection only if the Host and User columns in some user table record match the client hostname and username, and the client supplies the password specified in that record.

Host values in the user table may be specified as follows:

Because you can use IP wildcard values in the Host column (for example, '144.155.166.%' to match every host on a subnet), someone could try to exploit this capability by naming a host 144.155.166.somewhere.com. To foil such attempts, MySQL disallows matching on hostnames that start with digits and a dot. Thus, if you have a host named something like 1.2.foo.com, its name will never match the Host column of the grant tables. An IP wildcard value can match only IP numbers, not hostnames.

In the User column, wildcard characters are not allowed, but you can specify a blank value, which matches any name. If the user table entry that matches an incoming connection has a blank username, the user is considered to be an anonymous user with no name, not a user with the name that the client actually specified. This means that a blank username is used for all further access checking for the duration of the connection (that is, during Stage 2).

The Password column can be blank. This is not a wildcard and does not mean that any password matches. It means that the user must connect without specifying a password.

Non-blank Password values in the user table represent encrypted passwords. MySQL does not store passwords in plaintext form for anyone to see. Rather, the password supplied by a user who is attempting to connect is encrypted (using the PASSWORD() function). The encrypted password then is used during the connection process when checking whether the password is correct. (This is done without the encrypted password ever traveling over the connection.) From MySQL's point of view, the encrypted password is the REAL password, so you should not give anyone access to it! In particular, don't give non-administrative users read access to the tables in the mysql database!

From version 4.1 on, MySQL employs a stronger authentication method that has better password protection during the connection process than in earlier versions. It is secure even if TCP/IP packets are sniffed or the mysql database is captured. Password encryption is discussed further in section 5.5.9 Password Hashing in MySQL 4.1.

The following examples show how various combinations of Host and User values in the user table apply to incoming connections:

Host Value User Value Connections Matched by Entry
'thomas.loc.gov' 'fred' fred, connecting from thomas.loc.gov
'thomas.loc.gov' '' Any user, connecting from thomas.loc.gov
'%' 'fred' fred, connecting from any host
'%' '' Any user, connecting from any host
'%.loc.gov' 'fred' fred, connecting from any host in the loc.gov domain
'x.y.%' 'fred' fred, connecting from x.y.net, x.y.com, x.y.edu, and so on. (this is probably not useful)
'144.155.166.177' 'fred' fred, connecting from the host with IP address 144.155.166.177
'144.155.166.%' 'fred' fred, connecting from any host in the 144.155.166 class C subnet
'144.155.166.0/255.255.255.0' 'fred' Same as previous example

It is possible for the client hostname and username of an incoming connection to match more than one entry in the user table. The preceding set of examples demonstrates this: Several of the entries shown match a connection from thomas.loc.gov by fred.

When multiple matches are possible, the server must determine which of them to use. It resolves this issue as follows:

To see how this works, suppose that the user table looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| %         | root     | ...
| %         | jeffrey  | ...
| localhost | root     | ...
| localhost |          | ...
+-----------+----------+-

When the server reads in the table, it orders the entries with the most-specific Host values first. Literal hostnames and IP numbers are the most specific. The pattern '%' means ``any host'' and is least specific. Entries with the same Host value are ordered with the most-specific User values first (a blank User value means ``any user'' and is least specific). For the user table just shown, the result after sorting looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| localhost | root     | ...
| localhost |          | ...
| %         | jeffrey  | ...
| %         | root     | ...
+-----------+----------+-

When a client attempts to connect, the server looks through the sorted entries and uses the first match found. For a connection from localhost by jeffrey, two of the entries in the table match: the one with Host and User values of 'localhost' and '', and the one with values of '%' and 'jeffrey'. The 'localhost' entry appears first in sorted order, so that is the one the server uses.

Here is another example. Suppose that the user table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| %              | jeffrey  | ...
| thomas.loc.gov |          | ...
+----------------+----------+-

The sorted table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| thomas.loc.gov |          | ...
| %              | jeffrey  | ...
+----------------+----------+-

A connection by jeffrey from thomas.loc.gov is matched by the first entry, whereas a connection by jeffrey from whitehouse.gov is matched by the second.

It is a common misconception to think that, for a given username, all entries that explicitly name that user will be used first when the server attempts to find a match for the connection. This is simply not true. The previous example illustrates this, where a connection from thomas.loc.gov by jeffrey is first matched not by the entry containing 'jeffrey' as the User column value, but by the entry with no username! As a result, jeffrey will be authenticated as an anonymous user, even though he specified a username when connecting.

If you are able to connect to the server, but your privileges are not what you expect, you probably are being authenticated as some other account. To find out what account the server used to authenticate you, use the CURRENT_USER() function. It returns a value in user_name@host_name format that indicates the User and Host values from the matching user table record. Suppose that jeffrey connects and issues the following query:

mysql> SELECT CURRENT_USER();
+----------------+
| CURRENT_USER() |
+----------------+
| @localhost     |
+----------------+

The result shown here indicates that the matching user table entry had a blank User column value. In other words, the server is treating jeffrey as an anonymous user.

The CURRENT_USER() function is available as of MySQL 4.0.6. See section 12.8.3 Information Functions. Another thing you can do to diagnose authentication problems is to print out the user table and sort it by hand to see where the first match is being made.

5.5.6 Access Control, Stage 2: Request Verification

Once you establish a connection, the server enters Stage 2 of access control. For each request that comes in on the connection, the server determines what operation you want to perform, then checks whether you have sufficient privileges to do so. This is where the privilege columns in the grant tables come into play. These privileges can come from any of the user, db, host, tables_priv, or columns_priv tables. (You may find it helpful to refer to section 5.5.2 How the Privilege System Works, which lists the columns present in each of the grant tables.)

The user table grants privileges that are assigned to you on a global basis and that apply no matter what the current database is. For example, if the user table grants you the DELETE privilege, you can delete rows from any table in any database on the server host! In other words, user table privileges are superuser privileges. It is wise to grant privileges in the user table only to superusers such as database administrators. For other users, you should leave the privileges in the user table set to 'N' and grant privileges at more specific levels only. You can grant privileges for particular databases, tables, or columns.

The db and host tables grant database-specific privileges. Values in the scope columns of these tables can take the following forms:

The server reads in and sorts the db and host tables at the same time that it reads the user table. The server sorts the db table based on the Host, Db, and User scope columns, and sorts the host table based on the Host and Db scope columns. As with the user table, sorting puts the most-specific values first and least-specific values last, and when the server looks for matching entries, it uses the first match that it finds.

The tables_priv and columns_priv tables grant table-specific and column-specific privileges. Values in the scope columns of these tables can take the following form:

The server sorts the tables_priv and columns_priv tables based on the Host, Db, and User columns. This is similar to db table sorting, but simpler because only the Host column can contain wildcards.

The request verification process is described here. (If you are familiar with the access-checking source code, you will notice that the description here differs slightly from the algorithm used in the code. The description is equivalent to what the code actually does; it differs only to make the explanation simpler.)

For requests that require administrative privileges such as SHUTDOWN or RELOAD, the server checks only the user table entry because that is the only table that specifies administrative privileges. Access is granted if the entry allows the requested operation and denied otherwise. For example, if you want to execute mysqladmin shutdown but your user table entry doesn't grant the SHUTDOWN privilege to you, the server denies access without even checking the db or host tables. (They contain no Shutdown_priv column, so there is no need to do so.)

For database-related requests (INSERT, UPDATE, and so on), the server first checks the user's global (superuser) privileges by looking in the user table entry. If the entry allows the requested operation, access is granted. If the global privileges in the user table are insufficient, the server determines the user's database-specific privileges by checking the db and host tables:

  1. The server looks in the db table for a match on the Host, Db, and User columns. The Host and User columns are matched to the connecting user's hostname and MySQL username. The Db column is matched to the database that the user wants to access. If there is no entry for the Host and User, access is denied.
  2. If there is a matching db table entry and its Host column is not blank, that entry defines the user's database-specific privileges.
  3. If the matching db table entry's Host column is blank, it signifies that the host table enumerates which hosts should be allowed access to the database. In this case, a further lookup is done in the host table to find a match on the Host and Db columns. If no host table entry matches, access is denied. If there is a match, the user's database-specific privileges are computed as the intersection (not the union!) of the privileges in the db and host table entries; that is, the privileges that are 'Y' in both entries. (This way you can grant general privileges in the db table entry and then selectively restrict them on a host-by-host basis using the host table entries.)

After determining the database-specific privileges granted by the db and host table entries, the server adds them to the global privileges granted by the user table. If the result allows the requested operation, access is granted. Otherwise, the server successively checks the user's table and column privileges in the tables_priv and columns_priv tables, adds those to the user's privileges, and allows or denies access based on the result.

Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarized like this:

global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges

It may not be apparent why, if the global user entry privileges are initially found to be insufficient for the requested operation, the server adds those privileges to the database, table, and column privileges later. The reason is that a request might require more than one type of privilege. For example, if you execute an INSERT INTO ... SELECT statement, you need both the INSERT and the SELECT privileges. Your privileges might be such that the user table entry grants one privilege and the db table entry grants the other. In this case, you have the necessary privileges to perform the request, but the server cannot tell that from either table by itself; the privileges granted by the entries in both tables must be combined.

The host table is not affected by the GRANT or REVOKE statements, so it is unused in most MySQL installations. If you modify it directly, you can use it for some specialized purposes, such as to maintain a list of secure servers. For example, at TcX, the host table contains a list of all machines on the local network. These are granted all privileges.

You can also use the host table to indicate hosts that are not secure. Suppose that you have a machine public.your.domain that is located in a public area that you do not consider secure. You can allow access to all hosts on your network except that machine by using host table entries like this:

+--------------------+----+-
| Host               | Db | ...
+--------------------+----+-
| public.your.domain | %  | ... (all privileges set to 'N')
| %.your.domain      | %  | ... (all privileges set to 'Y')
+--------------------+----+-

Naturally, you should always test your entries in the grant tables (for example, by using SHOW GRANTS or mysqlaccess) to make sure that your access privileges are actually set up the way you think they are.

5.5.7 When Privilege Changes Take Effect

When mysqld starts, all grant table contents are read into memory and become effective for access control at that point.

When the server reloads the grant tables, privileges for existing client connections are affected as follows:

If you modify the grant tables using GRANT, REVOKE, or SET PASSWORD, the server notices these changes and reloads the grant tables into memory again immediately.

If you modify the grant tables directly using statements such as INSERT, UPDATE, or DELETE, your changes have no effect on privilege checking until you either restart the server or tell it to reload the tables. To reload the grant tables manually, issue a FLUSH PRIVILEGES statement or execute a mysqladmin flush-privileges or mysqladmin reload command.

If you change the grant tables directly but forget to reload them, your changes will have no effect until you restart the server. This may leave you wondering why your changes don't seem to make any difference!

5.5.8 Causes of Access denied Errors

If you encounter problems when you try to connect to the MySQL server, the following items describe some courses of action you can take to correct the problem.

5.5.9 Password Hashing in MySQL 4.1

MySQL user accounts are listed in the user table of the mysql database. Each MySQL account is assigned a password, although what is stored in the Password column of the user table is not the plaintext version of the password, but a hash value computed from it. Password hash values are computed by the PASSWORD() function.

MySQL uses passwords in two phases of client/server communication:

In other words, the server uses hash values during authentication when a client first attempts to connect. The server generates hash values if a connected client invokes the PASSWORD() function or uses a GRANT or SET PASSWORD statement to set or change a password.

The password hashing mechanism was updated in MySQL 4.1 to provide better security and to reduce the risk of passwords being intercepted. However, this new mechanism is understood only by the 4.1 server and 4.1 clients, which can result in some compatibility problems. A 4.1 client can connect to a pre-4.1 server, because the client understands both the old and new password hashing mechanisms. However, a pre-4.1 client that attempts to connect to a 4.1 server may run into difficulties. For example, a 4.0 mysql client that attempts to connect to a 4.1 server may fail with the following error message:

shell> mysql -h localhost -u root
Client does not support authentication protocol requested
by server; consider upgrading MySQL client

The following discussion describes the differences between the old and new password mechanisms, and what you should do if you upgrade your server to 4.1 but need to maintain backward compatibility with pre-4.1 clients. Additional information can be found in section A.2.3 Client does not support authentication protocol.

Note: This discussion contrasts 4.1 behavior with pre-4.1 behavior, but the 4.1 behavior described here actually begins with 4.1.1. MySQL 4.1.0 is an ``odd'' release because it has a slightly different mechanism than that implemented in 4.1.1 and up. Differences between 4.1.0 and more recent versions are described further in section 5.5.9.2 Password Hashing in MySQL 4.1.0.

Prior to MySQL 4.1, password hashes computed by the PASSWORD() function are 16 bytes long. Such hashes look like this:

mysql> SELECT PASSWORD('mypass');
+--------------------+
| PASSWORD('mypass') |
+--------------------+
| 6f8c114b58f2ce9e   |
+--------------------+

The Password column of the user table (in which these hashes are stored) also is 16 bytes long before MySQL 4.1.

As of MySQL 4.1, the PASSWORD() function has been modified to produce a longer 41-byte hash value:

mysql> SELECT PASSWORD('mypass');
+-----------------------------------------------+
| PASSWORD('mypass')                            |
+-----------------------------------------------+
| *43c8aa34cdc98eddd3de1fe9a9c2c2a9f92bb2098d75 |
+-----------------------------------------------+

Accordingly, the Password column in the user table also must be 41 bytes long to store these values:

A widened Password column can store password hashes in both the old and new formats. The format of any given password hash value can be determined two ways:

The longer password hash format has better cryptographic properties, and client authentication based on long hashes is more secure than that based on the older short hashes.

The differences between short and long password hashes are relevant both for how the server uses passwords during authentication and for how it generates password hashes for connected clients that perform password-changing operations.

The way in which the server uses password hashes during authentication is affected by the width of the Password column:

For short-hash accounts, the authentication process is actually a bit more secure for 4.1 clients than for older clients. In terms of security, the gradient from least to most secure is:

The way in which the server generates password hashes for connected clients is affected by the width of the Password column and by the --old-passwords option. A 4.1 server generates long hashes only if certain conditions are met: The Password column must be wide enough to hold long values and the --old-passwords option must not be given. These conditions apply as follows:

The purpose of the --old-passwords option is to allow you to maintain backward compatibility with pre-4.1 clients under circumstances where the server would otherwise generate long password hashes. The option doesn't affect authentication (4.1 clients can still use accounts that have long password hashes), but it does prevent creation of a long password hash in the user table as the result of a password-changing operation. Were that to occur, the account no longer could be used by pre-4.1 clients. Without the --old-passwords option, the following undesirable scenario is possible:

This scenario illustrates that, if you must support older pre-4.1 clients, it is dangerous to run a 4.1 server without using the --old-passwords option. By running the server with --old-passwords, password-changing operations will not generate long password hashes and thus do not cause accounts to become inaccessible to older clients. (Those clients cannot inadvertently lock themselves out by changing their password and ending up with a long password hash.)

The downside of the --old-passwords option is that any passwords you create or change will use short hashes, even for 4.1 clients. Thus, you lose the additional security provided by long password hashes. If you want to create an account that has a long hash (for example, for use by 4.1 clients), you must do so while running the server without --old-passwords.

The following scenarios are possible for running a 4.1 server:

Scenario 1: Short Password column in user table:

Scenario 2: Long Password column; server not started with --old-passwords option:

As indicated earlier, a danger in this scenario is that it is possible for accounts that have a short password hash to become inaccessible to pre-4.1 clients. A change to such an account's password made via GRANT, PASSWORD(), or SET PASSWORD results in the account being given a long password hash. From that point on, no pre-4.1 client can authenticate to that account until the client upgrades to 4.1.

To deal with this problem, you can change a password in a special way. For example, normally you use SET PASSWORD as follows to change an account password:

mysql> SET PASSWORD FOR 'some_user'@'some_host' = PASSWORD('mypass');

To change the password but create a short hash, use the OLD_PASSWORD() function instead:

mysql> SET PASSWORD FOR 'some_user'@'some_host' = OLD_PASSWORD('mypass');

OLD_PASSWORD() is useful for situations in which you explicitly want to generate a short hash.

Scenario 3: Long Password column; server started with --old-passwords option:

In this scenario, you cannot create accounts that have long password hashes, because the --old-passwords option prevents generation of long hashes. Also, if you create an account with a long hash before using the --old-passwords option, changing the account's password while --old-passwords is in effect results in the account being given a short password, causing it to lose the security benefits of a longer hash.

The disadvantages for these scenarios may be summarized as follows:

In scenario 1, you cannot take advantage of longer hashes that provide more secure authentication.

In scenario 2, accounts with short hashes become inaccessible to pre-4.1 clients if you change their passwords without explicitly using OLD_PASSWORD().

In scenario 3, --old-passwords prevents accounts with short hashes from becoming inaccessible, but password-changing operations cause accounts with long hashes to revert to short hashes, and you cannot change them back to long hashes while --old-passwords is in effect.

5.5.9.1 Implications of Password Hashing Changes for Application Programs

An upgrade to MySQL 4.1 can cause a compatibility issue for applications that use PASSWORD() to generate passwords for their own purposes. Applications really should not do this, because PASSWORD() should be used only to manage passwords for MySQL accounts. But some applications use PASSWORD() for their own purposes anyway.

If you upgrade to 4.1 and run the server under conditions where it generates long password hashes, an application that uses PASSWORD() for its own passwords will break. The recommended course of action is to modify the application to use another function, such as SHA1() or MD5(), to produce hashed values. If that is not possible, you can use the OLD_PASSWORD() function, which is provided to generate short hashes in the old format. But note that OLD_PASSWORD() may one day no longer be supported.

If the server is running under circumstances where it generates short hashes, OLD_PASSWORD() is available but is equivalent to PASSWORD().

5.5.9.2 Password Hashing in MySQL 4.1.0

Password hashing in MySQL 4.1.0 differs from hashing in 4.1.1 and up. The 4.1.0 differences are:

These differences make authentication in 4.1.0 incompatible with that of releases that follow it. If you have upgraded to MySQL 4.1.0, it is recommended that you upgrade to a newer version as soon as possible. After you do, reassign any long passwords in the user table so that they are compatible with the 41-byte format.

5.6 MySQL User Account Management

This section describes how to set up accounts for clients of your MySQL server. It discusses the following topics:

5.6.1 MySQL Usernames and Passwords

A MySQL account is defined in terms of a username and the client host or hosts from which the user can connect to the server. The account also has a password. There are several distinctions between the way usernames and passwords are used by MySQL and the way they are used by your operating system:

When you install MySQL, the grant tables are populated with an initial set of accounts. These accounts have names and access privileges that are described in section 2.9.3 Securing the Initial MySQL Accounts, which also discusses how to assign passwords to them. Thereafter, you normally set up, modify, and remove MySQL accounts using the GRANT and REVOKE statements. See section 13.5.1.2 GRANT and REVOKE Syntax.

When you connect to a MySQL server with a command-line client, you should specify the username and password for the account that you want to use:

shell> mysql --user=monty --password=guess db_name

If you prefer short options, the command looks like this:

shell> mysql -u monty -pguess db_name

There must be no space between the -p option and the following password value. See section 5.5.4 Connecting to the MySQL Server.

The preceding commands include the password value on the command line, which can be a security risk. See section 5.6.6 Keeping Your Password Secure. To avoid this, specify the --password or -p option without any following password value:

shell> mysql --user=monty --password db_name
shell> mysql -u monty -p db_name

Then the client program will print a prompt and wait for you to enter the password. (In these examples, db_name is not interpreted as a password, because it is separated from the preceding password option by a space.)

On some systems, the library call that MySQL uses to prompt for a password automatically limits the password to eight characters. That is a problem with the system library, not with MySQL. Internally, MySQL doesn't have any limit for the length of the password. To work around the problem, change your MySQL password to a value that is eight or fewer characters long, or put your password in an option file.

5.6.2 Adding New User Accounts to MySQL

You can create MySQL accounts in two ways:

The preferred method is to use GRANT statements, because they are more concise and less error-prone. GRANT is available as of MySQL 3.22.11; its syntax is described in section 13.5.1.2 GRANT and REVOKE Syntax.

Another option for creating accounts is to use one of several available third-party programs that offer capabilities for MySQL account administration. phpMyAdmin is one such program.

The following examples show how to use the mysql client program to set up new users. These examples assume that privileges are set up according to the defaults described in section 2.9.3 Securing the Initial MySQL Accounts. This means that to make changes, you must connect to the MySQL server as the MySQL root user, and the root account must have the INSERT privilege for the mysql database and the RELOAD administrative privilege.

First, use the mysql program to connect to the server as the MySQL root user:

shell> mysql --user=root mysql

If you have assigned a password to the root account, you'll also need to supply a --password or -p option for this mysql command and also for those later in this section.

After connecting to the server as root, you can add new accounts. The following statements use GRANT to set up four new accounts:

mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'localhost'
    ->     IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'%'
    ->     IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT RELOAD,PROCESS ON *.* TO 'admin'@'localhost';
mysql> GRANT USAGE ON *.* TO 'dummy'@'localhost';

The accounts created by these GRANT statements have the following properties:

As an alternative to GRANT, you can create the same accounts directly by issuing INSERT statements and then telling the server to reload the grant tables:

shell> mysql --user=root mysql
mysql> INSERT INTO user
    ->     VALUES('localhost','monty',PASSWORD('some_pass'),
    ->     'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user
    ->     VALUES('%','monty',PASSWORD('some_pass'),
    ->     'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user SET Host='localhost',User='admin',
    ->     Reload_priv='Y', Process_priv='Y';
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('localhost','dummy','');
mysql> FLUSH PRIVILEGES;

The reason for using FLUSH PRIVILEGES when you create accounts with INSERT is to tell the server to re-read the grant tables. Otherwise, the changes will go unnoticed until you restart the server. With GRANT, FLUSH PRIVILEGES is unnecessary.

The reason for using the PASSWORD() function with INSERT is to encrypt the password. The GRANT statement encrypts the password for you, so PASSWORD() is unnecessary.

The 'Y' values enable privileges for the accounts. Depending on your MySQL version, you may have to use a different number of 'Y' values in the first two INSERT statements. (Versions prior to 3.22.11 have fewer privilege columns, and versions from 4.0.2 on have more.) For the admin account, the more readable extended INSERT syntax using SET that is available starting with MySQL 3.22.11 is used.

In the INSERT statement for the dummy account, only the Host, User, and Password columns in the user table record are assigned values. None of the privilege columns are set explicitly, so MySQL assigns them all the default value of 'N'. This is equivalent to what GRANT USAGE does.

Note that to set up a superuser account, it is necessary only to create a user table entry with the privilege columns set to 'Y'. user table privileges are global, so no entries in any of the other grant tables are needed.

The next examples create three accounts and give them access to specific databases. Each of them has a username of custom and password of obscure.

To create the accounts with GRANT, use the following statements:

shell> mysql --user=root mysql
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
    ->     ON bankaccount.*
    ->     TO 'custom'@'localhost'
    ->     IDENTIFIED BY 'obscure';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
    ->     ON expenses.*
    ->     TO 'custom'@'whitehouse.gov'
    ->     IDENTIFIED BY 'obscure';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
    ->     ON customer.*
    ->     TO 'custom'@'server.domain'
    ->     IDENTIFIED BY 'obscure';

The three accounts can be used as follows:

To set up the custom accounts without GRANT, use INSERT statements as follows to modify the grant tables directly:

shell> mysql --user=root mysql
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('localhost','custom',PASSWORD('obscure'));
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('whitehouse.gov','custom',PASSWORD('obscure'));
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('server.domain','custom',PASSWORD('obscure'));
mysql> INSERT INTO db
    ->     (Host,Db,User,Select_priv,Insert_priv,
    ->     Update_priv,Delete_priv,Create_priv,Drop_priv)
    ->     VALUES('localhost','bankaccount','custom',
    ->     'Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
    ->     (Host,Db,User,Select_priv,Insert_priv,
    ->     Update_priv,Delete_priv,Create_priv,Drop_priv)
    ->     VALUES('whitehouse.gov','expenses','custom',
    ->     'Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
    ->     (Host,Db,User,Select_priv,Insert_priv,
    ->     Update_priv,Delete_priv,Create_priv,Drop_priv)
    ->     VALUES('server.domain','customer','custom',
    ->     'Y','Y','Y','Y','Y','Y');
mysql> FLUSH PRIVILEGES;

The first three INSERT statements add user table entries that allow the user custom to connect from the various hosts with the given password, but grant no global privileges (all privileges are set to the default value of 'N'). The next three INSERT statements add db table entries that grant privileges to custom for the bankaccount, expenses, and customer databases, but only when accessed from the proper hosts. As usual when you modify the grant tables directly, you tell the server to reload them with FLUSH PRIVILEGES so that the privilege changes take effect.

If you want to give a specific user access from all machines in a given domain (for example, mydomain.com), you can issue a GRANT statement that uses the `%' wildcard character in the host part of the account name:

mysql> GRANT ...
    ->     ON *.*
    ->     TO 'myname'@'%.mydomain.com'
    ->     IDENTIFIED BY 'mypass';

To do the same thing by modifying the grant tables directly, do this:

mysql> INSERT INTO user (Host,User,Password,...)
    ->     VALUES('%.mydomain.com','myname',PASSWORD('mypass'),...);
mysql> FLUSH PRIVILEGES;

5.6.3 Removing User Accounts from MySQL

To remove an account, use the DROP USER statement, which was added in MySQL 4.1.1. For older versions of MySQL, use DELETE instead. The account removal procedure is described in section 13.5.1.1 DROP USER Syntax.

5.6.4 Limiting Account Resources

Before MySQL 4.0.2, the only available method for limiting use of MySQL server resources is to set the max_user_connections system variable to a non-zero value. But that method is strictly global. It does not allow for management of individual accounts. Also, it limits only the number of simultaneous connections made using a single account, not what a client can do once connected. Both types of control are interest to many MySQL administrators, particularly those for Internet Service Providers.

Starting from MySQL 4.0.2, you can limit the following server resources for individual accounts:

Any statement that a client can issue counts against the query limit. Only statements that modify databases or tables count against the update limit.

An account in this context is a single record in the user table. Each account is uniquely identified by its User and Host column values.

As a prerequisite for using this feature, the user table in the mysql database must contain the resource-related columns. Resource limits are stored in the max_questions, max_updates, and max_connections columns. If your user table doesn't have these columns, it must be upgraded; see section 2.10.7 Upgrading the Grant Tables.

To set resource limits with a GRANT statement, use a WITH clause that names each resource to be limited and a per-hour count indicating the limit value. For example, to create a new account that can access the customer database, but only in a limited fashion, issue this statement:

mysql> GRANT ALL ON customer.* TO 'francis'@'localhost'
    ->     IDENTIFIED BY 'frank'
    ->     WITH MAX_QUERIES_PER_HOUR 20
    ->          MAX_UPDATES_PER_HOUR 10
    ->          MAX_CONNECTIONS_PER_HOUR 5;

The limit types need not all be named in the WITH clause, but those named can be present in any order. The value for each limit should be an integer representing a count per hour. If the GRANT statement has no WITH clause, the limits are each set to the default value of zero (that is, no limit).

To set or change limits for an existing account, use a GRANT USAGE statement at the global level (ON *.*). The following statement changes the query limit for francis to 100:

mysql> GRANT USAGE ON *.* TO 'francis'@'localhost'
    ->     WITH MAX_QUERIES_PER_HOUR 100;

This statement leaves the account's existing privileges unchanged and modifies only the limit values specified.

To remove an existing limit, set its value to zero. For example, to remove the limit on how many times per hour francis can connect, use this statement:

mysql> GRANT USAGE ON *.* TO 'francis'@'localhost'
    ->     WITH MAX_CONNECTIONS_PER_HOUR 0;

Resource-use counting takes place when any account has a non-zero limit placed on its use of any of the resources.

As the server runs, it counts the number of times each account uses resources. If an account reaches its limit on number of connections within the last hour, further connections for the account are rejected until that hour is up. Similarly, if the account reaches its limit on the number of queries or updates, further queries or updates are rejected until the hour is up. In all such cases, an appropriate error message is issued.

Resource counting is done per account, not per client. For example, if your account has a query limit of 50, you cannot increase your limit to 100 by making two simultaneous client connections to the server. Queries issued on both connections are counted together.

The current resource-use counts can be reset globally for all accounts, or individually for a given count:

5.6.5 Assigning Account Passwords

Passwords may be assigned from the command line by using the mysqladmin command:

shell> mysqladmin -u user_name -h host_name password "newpwd"

The account for which this command resets the password is the one with a user table record that matches user_name in the User column and the client host from which you connect in the Host column.

Another way to assign a password to an account is to issue a SET PASSWORD statement:

mysql> SET PASSWORD FOR 'jeffrey'@'%' = PASSWORD('biscuit');

Only users such as root with update access to the mysql database can change the password for other users. If you are not connected as an anonymous user, you can change your own password by omitting the FOR clause:

mysql> SET PASSWORD = PASSWORD('biscuit');

You can also use a GRANT USAGE statement at the global level (ON *.*) to assign a password to an account without affecting the account's current privileges:

mysql> GRANT USAGE ON *.* TO 'jeffrey'@'%' IDENTIFIED BY 'biscuit';

Although it is generally preferable to assign passwords using one of the preceding methods, you can also do so by modifying the user table directly:

When you assign an account a password using SET PASSWORD, INSERT, or UPDATE, you must use the PASSWORD() function to encrypt it. (The only exception is that you need not use PASSWORD() if the password is empty.) PASSWORD() is necessary because the user table stores passwords in encrypted form, not as plaintext. If you forget that fact, you are likely to set passwords like this:

shell> mysql -u root mysql
mysql> INSERT INTO user (Host,User,Password)
    -> VALUES('%','jeffrey','biscuit');
mysql> FLUSH PRIVILEGES;

The result is that the literal value 'biscuit' is stored as the password in the user table, not the encrypted value. When jeffrey attempts to connect to the server using this password, the value is encrypted and compared to the value stored in the user table. However, the stored value is the literal string 'biscuit', so the comparison fails and the server rejects the connection:

shell> mysql -u jeffrey -pbiscuit test
Access denied

If you set passwords using the GRANT ... IDENTIFIED BY statement or the mysqladmin password command, they both take care of encrypting the password for you. The PASSWORD() function is unnecessary.

Note: PASSWORD() encryption is different from Unix password encryption. See section 5.6.1 MySQL Usernames and Passwords.

5.6.6 Keeping Your Password Secure

On an administrative level, you should never grant access to the mysql.user table to any non-administrative accounts. Passwords in the user table are stored in encrypted form, but in versions of MySQL earlier than 4.1, knowing the encrypted password for an account makes it possible to connect to the server using that account.

When you run a client program to connect to the MySQL server, it is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed here, along with an assessment of the risks of each method:

All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly protected option file.

5.6.7 Using Secure Connections

Beginning with version 4.0.0, MySQL has support for secure (encrypted) connections between MySQL clients and the server using the Secure Sockets Layer (SSL) protocol. This section discusses how to use SSL connections. It also describes a way to set up SSH on Windows.

The standard configuration of MySQL is intended to be as fast as possible, so encrypted connections are not used by default. Doing so would make the client/server protocol much slower. Encrypting data is a CPU-intensive operation that requires the computer to do additional work and can delay other MySQL tasks. For applications that require the security provided by encrypted connections, the extra computation is warranted.

MySQL allows encryption to be enabled on a per-connection basis. You can choose a normal unencrypted connection or a secure encrypted SSL connection according the requirements of individual applications.

5.6.7.1 Basic SSL Concepts

To understand how MySQL uses SSL, it's necessary to explain some basic SSL and X509 concepts. People who are already familiar with them can skip this part.

By default, MySQL uses unencrypted connections between the client and the server. This means that someone with access to the network could watch all your traffic and look at the data being sent or received. They could even change the data while it is in transit between client and server. To improve security a little, you can compress client/server traffic by using the --compress option when invoking client programs. However, this will not foil a determined attacker.

When you need to move information over a network in a secure fashion, an unencrypted connection is unacceptable. Encryption is the way to make any kind of data unreadable. In fact, today's practice requires many additional security elements from encryption algorithms. They should resist many kind of known attacks such as changing the order of encrypted messages or replaying data twice.

SSL is a protocol that uses different encryption algorithms to ensure that data received over a public network can be trusted. It has mechanisms to detect any data change, loss, or replay. SSL also incorporates algorithms that provide identity verification using the X509 standard.

X509 makes it possible to identify someone on the Internet. It is most commonly used in e-commerce applications. In basic terms, there should be some company called a ``Certificate Authority'' (or CA) that assigns electronic certificates to anyone who needs them. Certificates rely on asymmetric encryption algorithms that have two encryption keys (a public key and a secret key). A certificate owner can show the certificate to another party as proof of identity. A certificate consists of its owner's public key. Any data encrypted with this public key can be decrypted only using the corresponding secret key, which is held by the owner of the certificate.

If you need more information about SSL, X509, or encryption, use your favorite Internet search engine to search for keywords in which you are interested.

5.6.7.2 Requirements

To use SSL connections between the MySQL server and client programs, your system must be able to support OpenSSL and your version of MySQL must be 4.0.0 or newer.

To get secure connections to work with MySQL, you must do the following:

  1. Install the OpenSSL library. We have tested MySQL with OpenSSL 0.9.6. If you need OpenSSL, visit http://www.openssl.org.
  2. When you configure MySQL, run the configure script with the --with-vio and --with-openssl options.
  3. Make sure that you have upgraded your grant tables to include the SSL-related columns in the mysql.user table. This is necessary if your grant tables date from a version prior to MySQL 4.0.0. The upgrade procedure is described in section 2.10.7 Upgrading the Grant Tables.
  4. To check whether a running mysqld server supports OpenSSL, examine the value of the have_openssl system variable:
    mysql> SHOW VARIABLES LIKE 'have_openssl';
    +---------------+-------+
    | Variable_name | Value |
    +---------------+-------+
    | have_openssl  | YES   |
    +---------------+-------+
    
    If the value is YES, the server supports OpenSSL connections.

5.6.7.3 Setting Up SSL Certificates for MySQL

Here is an example for setting up SSL certificates for MySQL:

DIR=`pwd`/openssl
PRIV=$DIR/private

mkdir $DIR $PRIV $DIR/newcerts
cp /usr/share/ssl/openssl.cnf $DIR
replace ./demoCA $DIR -- $DIR/openssl.cnf

# Create necessary files: $database, $serial and $new_certs_dir 
# directory (optional)

touch $DIR/index.txt
echo "01" > $DIR/serial

#
# Generation of Certificate Authority(CA)
#

openssl req -new -x509 -keyout $PRIV/cakey.pem -out $DIR/cacert.pem \
    -config $DIR/openssl.cnf

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# ................++++++
# .........++++++
# writing new private key to '/home/monty/openssl/private/cakey.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL admin
# Email Address []:

#
# Create server request and key
#
openssl req -new -keyout $DIR/server-key.pem -out \
    $DIR/server-req.pem -days 3600 -config $DIR/openssl.cnf

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# ..++++++
# ..........++++++
# writing new private key to '/home/monty/openssl/server-key.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL server
# Email Address []:
# 
# Please enter the following 'extra' attributes
# to be sent with your certificate request
# A challenge password []:
# An optional company name []:

#
# Remove the passphrase from the key (optional)
#

openssl rsa -in $DIR/server-key.pem -out $DIR/server-key.pem

#
# Sign server cert
#
openssl ca  -policy policy_anything -out $DIR/server-cert.pem \
    -config $DIR/openssl.cnf -infiles $DIR/server-req.pem

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Enter PEM pass phrase:
# Check that the request matches the signature
# Signature ok
# The Subjects Distinguished Name is as follows
# countryName           :PRINTABLE:'FI'
# organizationName      :PRINTABLE:'MySQL AB'
# commonName            :PRINTABLE:'MySQL admin'
# Certificate is to be certified until Sep 13 14:22:46 2003 GMT
# (365 days)
# Sign the certificate? [y/n]:y
# 
# 
# 1 out of 1 certificate requests certified, commit? [y/n]y
# Write out database with 1 new entries
# Data Base Updated

#
# Create client request and key
#
openssl req -new -keyout $DIR/client-key.pem -out \
    $DIR/client-req.pem -days 3600 -config $DIR/openssl.cnf

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# .....................................++++++
# .............................................++++++
# writing new private key to '/home/monty/openssl/client-key.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL user
# Email Address []:
# 
# Please enter the following 'extra' attributes
# to be sent with your certificate request
# A challenge password []:
# An optional company name []:

#
# Remove a passphrase from the key (optional)
#
openssl rsa -in $DIR/client-key.pem -out $DIR/client-key.pem

#
# Sign client cert
#

openssl ca  -policy policy_anything -out $DIR/client-cert.pem \
    -config $DIR/openssl.cnf -infiles $DIR/client-req.pem

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Enter PEM pass phrase:
# Check that the request matches the signature
# Signature ok
# The Subjects Distinguished Name is as follows
# countryName           :PRINTABLE:'FI'
# organizationName      :PRINTABLE:'MySQL AB'
# commonName            :PRINTABLE:'MySQL user'
# Certificate is to be certified until Sep 13 16:45:17 2003 GMT
# (365 days)
# Sign the certificate? [y/n]:y
# 
# 
# 1 out of 1 certificate requests certified, commit? [y/n]y
# Write out database with 1 new entries
# Data Base Updated

#
# Create a my.cnf file that you can use to test the certificates
#

cnf=""
cnf="$cnf [client]"
cnf="$cnf ssl-ca=$DIR/cacert.pem"
cnf="$cnf ssl-cert=$DIR/client-cert.pem"
cnf="$cnf ssl-key=$DIR/client-key.pem"
cnf="$cnf [mysqld]"
cnf="$cnf ssl-ca=$DIR/cacert.pem"
cnf="$cnf ssl-cert=$DIR/server-cert.pem"
cnf="$cnf ssl-key=$DIR/server-key.pem"
echo $cnf | replace " " '
' > $DIR/my.cnf

To test SSL connections, start the server as follows, where $DIR is the pathname to the directory where the sample `my.cnf' option file is located:

shell> mysqld --defaults-file=$DIR/my.cnf &

Then invoke a client program using the same option file:

shell> mysql --defaults-file=$DIR/my.cnf

If you have a MySQL source distribution, you can also test your setup by modifying the preceding `my.cnf' file to refer to the demonstration certificate and key files in the `SSL' directory of the distribution.

5.6.7.4 SSL GRANT Options

MySQL can check X509 certificate attributes in addition to the usual authentication that is based on the username and password. To specify SSL-related options for a MySQL account, use the REQUIRE clause of the GRANT statement. See section 13.5.1.2 GRANT and REVOKE Syntax.

There are different possibilities for limiting connection types for an account:

The SUBJECT, ISSUER, and CIPHER options can be combined in the REQUIRE clause like this:

mysql> GRANT ALL PRIVILEGES ON test.* TO 'root'@'localhost'
    -> IDENTIFIED BY 'goodsecret'
    -> REQUIRE SUBJECT '/C=EE/ST=Some-State/L=Tallinn/
       O=MySQL demo client certificate/
       CN=Tonu Samuel/Email=tonu@example.com'
    -> AND ISSUER '/C=FI/ST=Some-State/L=Helsinki/
       O=MySQL Finland AB/CN=Tonu Samuel/Email=tonu@example.com'
    -> AND CIPHER 'EDH-RSA-DES-CBC3-SHA';

Note that the SUBJECT and ISSUER values each should be entered as a single string.

Starting from MySQL 4.0.4, the AND keyword is optional between REQUIRE options.

The order of the options does not matter, but no option can be specified twice.

5.6.7.5 SSL Command-Line Options

The following list describes options that are used for specifying the use of SSL, certificate files, and key files. These options are available beginning with MySQL 4.0. They may be given on the command line or in an option file.

--ssl
For the server, this option specifies that the server allows SSL connections. For a client program, it allows the client to connect to the server using SSL. This option is not sufficient in itself to cause an SSL connection to be used. You must also specify the --ssl-ca, --ssl-cert, and --ssl-key options. This option is more often used in its opposite form to indicate that SSL should not be used. To do this, specify the option as --skip-ssl or --ssl=0. Note that use of --ssl doesn't require an SSL connection. For example, if the server or client is compiled without SSL support, a normal unencrypted connection will be used. The secure way to ensure that an SSL connection will be used is to create an account on the server that includes a REQUIRE SSL clause in the GRANT statement. Then use this account to connect to the server, with both a server and client that have SSL support enabled.
--ssl-ca=file_name
The path to a file with a list of trusted SSL CAs.
--ssl-capath=directory_name
The path to a directory that contains trusted SSL CA certificates in pem format.
--ssl-cert=file_name
The name of the SSL certificate file to use for establishing a secure connection.
--ssl-cipher=cipher_list
A list of allowable ciphers to use for SSL encryption. cipher_list has the same format as the openssl ciphers command. Example: --ssl-cipher=ALL:-AES:-EXP
--ssl-key=file_name
The name of the SSL key file to use for establishing a secure connection.

5.6.7.6 Connecting to MySQL Remotely from Windows with SSH

Here is a note about how to connect to get a secure connection to remote MySQL server with SSH (by David Carlson dcarlson@mplcomm.com):

  1. Install an SSH client on your Windows machine. As a user, the best non-free one I've found is from SecureCRT from http://www.vandyke.com/. Another option is f-secure from http://www.f-secure.com/. You can also find some free ones on Google at http://directory.google.com/Top/Computers/Security/Products_and_Tools/Cryptography/SSH/Clients/Windows/.
  2. Start your Windows SSH client. Set Host_Name = yourmysqlserver_URL_or_IP. Set userid=your_userid to log in to your server. This userid value may not be the same as the username of your MySQL account.
  3. Set up port forwarding. Either do a remote forward (Set local_port: 3306, remote_host: yourmysqlservername_or_ip, remote_port: 3306 ) or a local forward (Set port: 3306, host: localhost, remote port: 3306).
  4. Save everything, otherwise you'll have to redo it the next time.
  5. Log in to your server with the SSH session you just created.
  6. On your Windows machine, start some ODBC application (such as Access).
  7. Create a new file in Windows and link to MySQL using the ODBC driver the same way you normally do, except type in localhost for the MySQL host server, not yourmysqlservername.

You should now have an ODBC connection to MySQL, encrypted using SSH.

5.7 Disaster Prevention and Recovery

This section discusses how to make database backups (full and incremental) and how to perform table maintenance. The syntax of the SQL statements described here is given in section 13.5 Database Administration Statements. Much of the information here pertains primarily to MyISAM tables. InnoDB backup procedures are given in section 15.9 Backing Up and Recovering an InnoDB Database.

5.7.1 Database Backups

Because MySQL tables are stored as files, it is easy to do a backup. To get a consistent backup, do a LOCK TABLES on the relevant tables, followed by FLUSH TABLES for the tables. See section 13.4.5 LOCK TABLES and UNLOCK TABLES Syntax and section 13.5.5.2 FLUSH Syntax. You need only a read lock; this allows other clients to continue to query the tables while you are making a copy of the files in the database directory. The FLUSH TABLES statement is needed to ensure that the all active index pages are written to disk before you start the backup.

If you want to make an SQL-level backup of a table, you can use SELECT INTO ... OUTFILE or BACKUP TABLE. For SELECT INTO ... OUTFILE, the output file cannot already exist. For BACKUP TABLE, the same is true as of MySQL 3.23.56 and 4.0.12, because this would be a security risk. See section 13.1.7 SELECT Syntax and section 13.5.2.2 BACKUP TABLE Syntax.

Another way to back up a database is to use the mysqldump program or the mysqlhotcopy script. See section 8.8 The mysqldump Database Backup Program and section 8.9 The mysqlhotcopy Database Backup Program.

  1. Do a full backup of your database:
    shell> mysqldump --tab=/path/to/some/dir --opt db_name
    
    Or:
    shell> mysqlhotcopy db_name /path/to/some/dir
    
    You can also simply copy all table files (`*.frm', `*.MYD', and `*.MYI' files) as long as the server isn't updating anything. The mysqlhotcopy script uses this method. (But note that these methods will not work if your database contains InnoDB tables. InnoDB does not store table contents in database directories, and mysqlhotcopy works only for MyISAM and ISAM tables.)
  2. Stop mysqld if it's running, then start it with the --log-bin[=file_name] option. See section 5.9.4 The Binary Log. The binary log files provide you with the information you need to replicate changes to the database that are made subsequent to the point at which you executed mysqldump.

For InnoDB tables, it's possible to perform an online backup that takes no locks on tables; see section 8.8 The mysqldump Database Backup Program

MySQL supports incremental backups: You need to start the server with the --log-bin option to enable binary logging; see section 5.9.4 The Binary Log. At the moment you want to make an incremental backup (containing all changes that happened since the last full or incremental backup), you should rotate the binary log by using FLUSH LOGS. This done, you need to copy to the backup location all binary logs which range from the one of the moment of the last full or incremental backup to the last but one. These binary logs are the incremental backup; at restore time, you apply them as explained further below. The next time you do a full backup, you should also rotate the binary log using FLUSH LOGS, mysqldump --flush-logs, or mysqlhotcopy --flushlogs. See section 8.8 The mysqldump Database Backup Program and section 8.9 The mysqlhotcopy Database Backup Program.

If your MySQL server is a slave replication server, then regardless of the backup method you choose, you should also back up the `master.info' and `relay-log.info' files when you back up your slave's data. These files are always needed to resume replication after you restore the slave's data. If your slave is subject to replicating LOAD DATA INFILE commands, you should also back up any `SQL_LOAD-*' files that may exist in the directory specified by the --slave-load-tmpdir option. (This location defaults to the value of the tmpdir variable if not specified.) The slave needs these files to resume replication of any interrupted LOAD DATA INFILE operations.

If you have to restore MyISAM tables, try to recover them using REPAIR TABLE or myisamchk -r first. That should work in 99.9% of all cases. If myisamchk fails, try the following procedure. Note that it will work only if you have enabled binary logging by starting MySQL with the --log-bin option; see section 5.9.4 The Binary Log.

  1. Restore the original mysqldump backup, or binary backup.
  2. Execute the following command to re-run the updates in the binary logs:
    shell> mysqlbinlog hostname-bin.[0-9]* | mysql
    
    In your case, you may want to re-run only certain binary logs, from certain positions (usually you want to re-run all binary logs from the date of the restored backup, excepting possibly some incorrect queries). See section 8.5 The mysqlbinlog Binary Log Utility for more information on the mysqlbinlog utility and how to use it. If you are using the update logs instead (which is a deprecated feature removed in MySQL 5.0), you can process their contents like this:
    shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql
    
    ls is used to sort the update log filenames into the right order.

You can also do selective backups of individual files:

If you have performance problems with your server while making backups, one strategy that can help is to set up replication and perform backups on the slave rather than on the master. See section 6.1 Introduction to Replication.

If you are using a Veritas filesystem, you can make a backup like this:

  1. From a client program, execute FLUSH TABLES WITH READ LOCK.
  2. From another shell, execute mount vxfs snapshot.
  3. From the first client, execute UNLOCK TABLES.
  4. Copy files from the snapshot.
  5. Unmount the snapshot.

5.7.2 Table Maintenance and Crash Recovery

The following text discusses how to use myisamchk to check or repair MyISAM tables (tables with `.MYI' and `.MYD' files). The same concepts apply to using isamchk to check or repair ISAM tables (tables with `.ISM' and `.ISD' files). See section 14 MySQL Storage Engines and Table Types.

You can use the myisamchk utility to get information about your database tables or to check, repair, or optimize them. The following sections describe how to invoke myisamchk (including a description of its options), how to set up a table maintenance schedule, and how to use myisamchk to perform its various functions.

Even though table repair with myisamchk is quite secure, it's always a good idea to make a backup before doing a repair (or any maintenance operation that could make a lot of changes to a table)

myisamchk operations that affect indexes can cause FULLTEXT indexes to be rebuilt with full-text parameters that are incompatible with the values used by the MySQL server. To avoid this, read the instructions in section 5.7.2.2 General Options for myisamchk.

In many cases, you may find it simpler to do MyISAM table maintenance using the SQL statements that perform operations that myisamchk can do:

These statements were introduced in different versions, but all are available from MySQL 3.23.14 on. See section 13.5.2.1 ANALYZE TABLE Syntax, section 13.5.2.3 CHECK TABLE Syntax, section 13.5.2.5 OPTIMIZE TABLE Syntax, and section 13.5.2.6 REPAIR TABLE Syntax. The statements can be used directly, or by means of the mysqlcheck client program, which provides a command-line interface to them.

One advantage of these statements over myisamchk is that the server does all the work. With myisamchk, you must make sure that the server does not use the tables at the same time. Otherwise, there can be unwanted interaction betweeen myisamchk and the server.

5.7.2.1 myisamchk Invocation Syntax

Invoke myisamchk like this:

shell> myisamchk [options] tbl_name

The options specify what you want myisamchk to do. They are described in the following sections. You can also get a list of options by invoking myisamchk --help.

With no options, myisamchk simply checks your table as the default operation. To get more information or to tell myisamchk to take corrective action, specify options as described in the following discussion.

tbl_name is the database table you want to check or repair. If you run myisamchk somewhere other than in the database directory, you must specify the path to the database directory, because myisamchk has no idea where the database is located. In fact, myisamchk doesn't actually care whether the files you are working on are located in a database directory. You can copy the files that correspond to a database table into some other location and perform recovery operations on them there.

You can name several tables on the myisamchk command line if you wish. You can also specify a table by naming its index file (the file with the `.MYI' suffix). This allows you to specify all tables in a directory by using the pattern `*.MYI'. For example, if you are in a database directory, you can check all the MyISAM tables in that directory like this:

shell> myisamchk *.MYI

If you are not in the database directory, you can check all the tables there by specifying the path to the directory:

shell> myisamchk /path/to/database_dir/*.MYI

You can even check all tables in all databases by specifying a wildcard with the path to the MySQL data directory:

shell> myisamchk /path/to/datadir/*/*.MYI

The recommended way to quickly check all MyISAM and ISAM tables is:

shell> myisamchk --silent --fast /path/to/datadir/*/*.MYI
shell> isamchk --silent /path/to/datadir/*/*.ISM

If you want to check all MyISAM and ISAM tables and repair any that are corrupted, you can use the following commands:

shell> myisamchk --silent --force --fast --update-state \
          -O key_buffer=64M -O sort_buffer=64M \
          -O read_buffer=1M -O write_buffer=1M \
          /path/to/datadir/*/*.MYI
shell> isamchk --silent --force -O key_buffer=64M \
          -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M \
          /path/to/datadir/*/*.ISM

These commands assume that you have more than 64MB free. For more information about memory allocation with myisamchk, see section 5.7.2.6 myisamchk Memory Usage.

You must ensure that no other program is using the tables while you are running myisamchk. Otherwise, when you run myisamchk, it may display the following error message:

warning: clients are using or haven't closed the table properly

This means that you are trying to check a table that has been updated by another program (such as the mysqld server) that hasn't yet closed the file or that has died without closing the file properly.

If mysqld is running, you must force it to flush any table modifications that are still buffered in memory by using FLUSH TABLES. You should then ensure that no one is using the tables while you are running myisamchk. The easiest way to avoid this problem is to use CHECK TABLE instead of myisamchk to check tables.

5.7.2.2 General Options for myisamchk

The options described in this section can be used for any type of table maintenance operation performed by myisamchk. The sections following this one describe options that pertain only to specific operations, such as table checking or repairing.

--help, -?
Display a help message and exit.
--debug=debug_options, -# debug_options
Write a debugging log. The debug_options string often is 'd:t:o,file_name'.
--silent, -s
Silent mode. Write output only when errors occur. You can use -s twice (-ss) to make myisamchk very silent.
--verbose, -v
Verbose mode. Print more information. This can be used with -d and -e. Use -v multiple times (-vv, -vvv) for even more output.
--version, -V
Display version information and exit.
--wait, -w
Instead of terminating with an error if the table is locked, wait until the table is unlocked before continuing. Note that if you are running mysqld with the --skip-external-locking option, the table can be locked only by another myisamchk command.

You can also set the following variables by using --var_name=value options:

Variable Default Value
decode_bits 9
ft_max_word_len version-dependent
ft_min_word_len 4
ft_stopword_file built-in list
key_buffer_size 523264
myisam_block_size 1024
read_buffer_size 262136
sort_buffer_size 2097144
sort_key_blocks 16
write_buffer_size 262136

It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. However, this syntax is deprecated as of MySQL 4.0.

The possible myisamchk variables and their default values can be examined with myisamchk --help:

sort_buffer_size is used when the keys are repaired by sorting keys, which is the normal case when you use --recover.

key_buffer_size is used when you are checking the table with --extend-check or when the keys are repaired by inserting keys row by row into the table (like when doing normal inserts). Repairing through the key buffer is used in the following cases:

Repairing through the key buffer takes much less disk space than using sorting, but is also much slower.

If you want a faster repair, set the key_buffer_size and sort_buffer_size variables to about 25% of your available memory. You can set both variables to large values, because only one of them is used at a time.

myisam_block_size is the size used for index blocks. It is available as of MySQL 4.0.0.

The ft_min_word_len and ft_max_word_len variables are available as of MySQL 4.0.0. ft_stopword_file is available as of MySQL 4.0.19.

ft_min_word_len and ft_max_word_len indicate the minimum and maximum word length for FULLTEXT indexes. ft_stopword_file names the stopword file. These need to be set under the following circumstances.

If you use myisamchk to perform an operation that modifies table indexes (such as repair or analyze), the FULLTEXT indexes are rebuilt using the default full-text parameter values for minimum and maximum word length and the stopword file unless you specify otherwise. This can result in queries failing.

The problem occurs because these parameters are known only by the server. They are not stored in MyISAM index files. To avoid the problem if you have modified the minimum or maximum word length or the stopword file in the server, specify the same ft_min_word_len, ft_max_word_len, and ft_stopword_file values to myisamchk that you use for mysqld. For example, if you have set the minimum word length to 3, you can repair a table with myisamchk like this:

shell> myisamchk --recover --ft_min_word_len=3 tbl_name.MYI

To ensure that myisamchk and the server use the same values for full-text parameters, you can place each one in both the [mysqld] and [myisamchk] sections of an option file:

[mysqld]
ft_min_word_len=3

[myisamchk]
ft_min_word_len=3

An alternative to using myisamchk is to use the REPAIR TABLE, ANALYZE TABLE, OPTIMIZE TABLE, or ALTER TABLE. These statements are performed by the server, which knows the proper full-text parameter values to use.

5.7.2.3 Check Options for myisamchk

myisamchk supports the following options for table checking operations:

--check, -c
Check the table for errors. This is the default operation if you specify no option that selects an operation type explicitly.
--check-only-changed, -C
Check only tables that have changed since the last check.
--extend-check, -e
Check the table very thoroughly. This is quite slow if the table has many indexes. This option should only be used in extreme cases. Normally, myisamchk or myisamchk --medium-check should be able to determine whether there are any errors in the table. If you are using --extend-check and have plenty of memory, setting the key_buffer_size variable to a large value will help the repair operation run faster.
--fast, -F
Check only tables that haven't been closed properly.
--force, -f
Do a repair operation automatically if myisamchk finds any errors in the table. The repair type is the same as that specified with the --repair or -r option.
--information, -i
Print informational statistics about the table that is checked.
--medium-check, -m
Do a check that is faster than an --extend-check operation. This finds only 99.99% of all errors, which should be good enough in most cases.
--read-only, -T
Don't mark the table as checked. This is useful if you use myisamchk to check a table that is in use by some other application that doesn't use locking, such as mysqld when run with the --skip-external-locking option.
--update-state, -U
Store information in the `.MYI' file to indicate when the table was checked and whether the table crashed. This should be used to get full benefit of the --check-only-changed option, but you shouldn't use this option if the mysqld server is using the table and you are running it with the --skip-external-locking option.

5.7.2.4 Repair Options for myisamchk

myisamchk supports the following options for table repair operations:

--backup, -B
Make a backup of the `.MYD' file as `file_name-time.BAK'
--character-sets-dir=path
The directory where character sets are installed. See section 5.8.1 The Character Set Used for Data and Sorting.
--correct-checksum
Correct the checksum information for the table.
--data-file-length=#, -D #
Maximum length of the data file (when re-creating data file when it's ``full'').
--extend-check, -e
Do a repair that tries to recover every possible row from the data file. Normally this will also find a lot of garbage rows. Don't use this option unless you are totally desperate.
--force, -f
Overwrite old temporary files (files with names like `tbl_name.TMD') instead of aborting.
--keys-used=#, -k #
For myisamchk, the option value indicates which indexes to update. Each binary bit of the option value corresponds to a table index, where the first index is bit 0. For isamchk, the option value indicates that only the first # of the table indexes should be updated. In either case, an option value of 0 disables updates to all indexes, which can be used to get faster inserts. Deactivated indexes can be reactivated by using myisamchk -r or (isamchk -r).
--no-symlinks, -l
Do not follow symbolic links. Normally myisamchk repairs the table that a symlink points to. This option doesn't exist as of MySQL 4.0, because versions from 4.0 on will not remove symlinks during repair operations.
--parallel-recover, -p
Uses the same technique as -r and -n, but creates all the keys in parallel, using different threads. This option was added in MySQL 4.0.2. This is alpha code. Use at your own risk!
--quick, -q
Achieve a faster repair by not modifying the data file. You can specify this option twice to force myisamchk to modify the original data file in case of duplicate keys.
--recover, -r
Do a repair that can fix almost any problem except unique keys that aren't unique (which is an extremely unlikely error with ISAM/MyISAM tables). If you want to recover a table, this is the option to try first. You should try -o only if myisamchk reports that the table can't be recovered by -r. (In the unlikely case that -r fails, the data file is still intact.) If you have lots of memory, you should increase the value of sort_buffer_size.
--safe-recover, -o
Do a repair using an old recovery method that reads through all rows in order and updates all index trees based on the rows found. This is an order of magnitude slower than -r, but can handle a couple of very unlikely cases that -r cannot. This recovery method also uses much less disk space than -r. Normally, you should repair first with -r, and then with -o only if -r fails. If you have lots of memory, you should increase the value of key_buffer_size.
--set-character-set=name
Change the character set used by the table indexes.
--sort-recover, -n
Force myisamchk to use sorting to resolve the keys even if the temporary files should be very big.
--tmpdir=path, -t path
Path of the directory to be used for storing temporary files. If this is not set, myisamchk uses the value of the TMPDIR environment variable. Starting from MySQL 4.1, tmpdir can be set to a list of directory paths that will be used successively in round-robin fashion for creating temporary files. The separator character between directory names should be colon (`:') on Unix and semicolon (`;') on Windows, NetWare, and OS/2.
--unpack, -u
Unpack a table that was packed with myisampack.

5.7.2.5 Other Options for myisamchk

myisamchk supports the following options for actions other than table checks and repairs:

--analyze, -a
Analyze the distribution of keys. This improves join performance by enabling the join optimizer to better choose the order in which to join the tables and which keys it should use. To obtain information about the distribution, use a myisamchk --description --verbose tbl_name command or the SHOW KEYS FROM tbl_name statement.
--description, -d
Print some descriptive information about the table.
--set-auto-increment[=value], -A[value]
Force AUTO_INCREMENT numbering for new records to start at the given value (or higher, if there are already records with AUTO_INCREMENT values this large). If value is not specified, AUTO_INCREMENT number for new records begins with the largest value currently in the table, plus one.
--sort-index, -S
Sort the index tree blocks in high-low order. This optimizes seeks and makes table scanning by key faster.
--sort-records=#, -R #
Sort records according to a particular index. This makes your data much more localized and may speed up range-based SELECT and ORDER BY operations that use this index. (The first time you use this option to sort a table, it may be very slow.) To determine a table's index numbers, use SHOW KEYS, which displays a table's indexes in the same order that myisamchk sees them. Indexes are numbered beginning with 1.

5.7.2.6 myisamchk Memory Usage

Memory allocation is important when you run myisamchk. myisamchk uses no more memory than you specify with the -O options. If you are going to use myisamchk on very large tables, you should first decide how much memory you want it to use. The default is to use only about 3MB to perform repairs. By using larger values, you can get myisamchk to operate faster. For example, if you have more than 32MB RAM, you could use options such as these (in addition to any other options you might specify):

shell> myisamchk -O sort=16M -O key=16M -O read=1M -O write=1M ...

Using -O sort=16M should probably be enough for most cases.

Be aware that myisamchk uses temporary files in TMPDIR. If TMPDIR points to a memory filesystem, you may easily get out of memory errors. If this happens, set TMPDIR to point at some directory located on a filesystem with more space and run myisamchk again.

When repairing, myisamchk will also need a lot of disk space:

If you have a problem with disk space during repair, you can try to use --safe-recover instead of --recover.

5.7.2.7 Using myisamchk for Crash Recovery

If you run mysqld with --skip-external-locking (which is the default on some systems, such as Linux), you can't reliably use myisamchk to check a table when mysqld is using the same table. If you can be sure that no one is accessing the tables through mysqld while you run myisamchk, you only have to do mysqladmin flush-tables before you start checking the tables. If you can't guarantee this, then you must stop mysqld while you check the tables. If you run myisamchk while mysqld is updating the tables, you may get a warning that a table is corrupt even when it isn't.

If you are not using --skip-external-locking, you can use myisamchk to check tables at any time. While you do this, all clients that try to update the table will wait until myisamchk is ready before continuing.

If you use myisamchk to repair or optimize tables, you must always ensure that the mysqld server is not using the table (this also applies if you are using --skip-external-locking). If you don't take down mysqld, you should at least do a mysqladmin flush-tables before you run myisamchk. Your tables may become corrupted if the server and myisamchk access the tables simultaneously.

This section describes how to check for and deal with data corruption in MySQL databases. If your tables get corrupted frequently you should try to find the reason why. See section A.4.2 What to Do If MySQL Keeps Crashing.

The MyISAM table section contains reason for why a table could be corrupted. See section 14.1.4 MyISAM Table Problems.

When performing crash recovery, it is important to understand that each MyISAM table tbl_name in a database corresponds to three files in the database directory:

File Purpose
`tbl_name.frm' Definition (format) file
`tbl_name.MYD' Data file
`tbl_name.MYI' Index file

Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.

myisamchk works by creating a copy of the `.MYD' data file row by row. It ends the repair stage by removing the old `.MYD' file and renaming the new file to the original file name. If you use --quick, myisamchk does not create a temporary `.MYD' file, but instead assumes that the `.MYD' file is correct and only generates a new index file without touching the `.MYD' file. This is safe, because myisamchk automatically detects whether the `.MYD' file is corrupt and aborts the repair if it is. You can also specify the --quick option twice to myisamchk. In this case, myisamchk does not abort on some errors (such as duplicate-key errors) but instead tries to resolve them by modifying the `.MYD' file. Normally the use of two --quick options is useful only if you have too little free disk space to perform a normal repair. In this case, you should at least make a backup before running myisamchk.

5.7.2.8 How to Check MyISAM Tables for Errors

To check a MyISAM table, use the following commands:

myisamchk tbl_name
This finds 99.99% of all errors. What it can't find is corruption that involves only the data file (which is very unusual). If you want to check a table, you should normally run myisamchk without options or with either the -s or --silent option.
myisamchk -m tbl_name
This finds 99.999% of all errors. It first checks all index entries for errors and then reads through all rows. It calculates a checksum for all keys in the rows and verifies that the checksum matches the checksum for the keys in the index tree.
myisamchk -e tbl_name
This does a complete and thorough check of all data (-e means ``extended check''). It does a check-read of every key for each row to verify that they indeed point to the correct row. This may take a long time for a large table that has many indexes. Normally, myisamchk stops after the first error it finds. If you want to obtain more information, you can add the --verbose (-v) option. This causes myisamchk to keep going, up through a maximum of 20 errors.
myisamchk -e -i tbl_name
Like the previous command, but the -i option tells myisamchk to print some informational statistics, too.

In most cases, a simple myisamchk with no arguments other than the table name is sufficient to check a table.

5.7.2.9 How to Repair Tables

The discussion in this section describes how to use myisamchk on MyISAM tables (extensions `.MYI' and `.MYD'). If you are using ISAM tables (extensions `.ISM' and `.ISD'), you should use isamchk instead; the concepts are similar.

If you are using MySQL 3.23.16 and above, you can (and should) use the CHECK TABLE and REPAIR TABLE statements to check and repair MyISAM tables. See section 13.5.2.3 CHECK TABLE Syntax and section 13.5.2.6 REPAIR TABLE Syntax.

The symptoms of a corrupted table include queries that abort unexpectedly and observable errors such as these:

To get more information about the error you can run perror ###, where ### is the error number. The following example shows how to use perror to find the meanings for the most common error numbers that indicate a problem with a table:

shell> perror 126 127 132 134 135 136 141 144 145
126 = Index file is crashed / Wrong file format
127 = Record-file is crashed
132 = Old database file
134 = Record was already deleted (or record file crashed)
135 = No more room in record file
136 = No more room in index file
141 = Duplicate unique key or constraint on write or update
144 = Table is crashed and last repair failed
145 = Table was marked as crashed and should be repaired

Note that error 135 (no more room in record file) and error 136 (no more room in index file) are not errors that can be fixed by a simple repair. In this case, you have to use ALTER TABLE to increase the MAX_ROWS and AVG_ROW_LENGTH table option values:

ALTER TABLE tbl_name MAX_ROWS=xxx AVG_ROW_LENGTH=yyy;

If you don't know the current table option values, use SHOW CREATE TABLE tbl_name.

For the other errors, you must repair your tables. myisamchk can usually detect and fix most problems that occur.

The repair process involves up to four stages, described here. Before you begin, you should change location to the database directory and check the permissions of the table files. On Unix, make sure that they are readable by the user that mysqld runs as (and to you, because you need to access the files you are checking). If it turns out you need to modify files, they must also be writable by you.

The options that you can use for table maintenance with myisamchk and isamchk are described in several of the earlier subsections of section 5.7.2 Table Maintenance and Crash Recovery.

The following section is for the cases where the above command fails or if you want to use the extended features that myisamchk and isamchk provide.

If you are going to repair a table from the command line, you must first stop the mysqld server. Note that when you do mysqladmin shutdown on a remote server, the mysqld server will still be alive for a while after mysqladmin returns, until all queries are stopped and all keys have been flushed to disk.

Stage 1: Checking your tables

Run myisamchk *.MYI or myisamchk -e *.MYI if you have more time. Use the -s (silent) option to suppress unnecessary information.

If the mysqld server is down, you should use the --update-state option to tell myisamchk to mark the table as 'checked'.

You have to repair only those tables for which myisamchk announces an error. For such tables, proceed to Stage 2.

If you get weird errors when checking (such as out of memory errors), or if myisamchk crashes, go to Stage 3.

Stage 2: Easy safe repair

Note: If you want a repair operation to go much faster, you should set the values of the sort_buffer_size and key_buffer_size variables each to about 25% of your available memory when running myisamchk or isamchk.

First, try myisamchk -r -q tbl_name (-r -q means ``quick recovery mode''). This will attempt to repair the index file without touching the data file. If the data file contains everything that it should and the delete links point at the correct locations within the data file, this should work, and the table is fixed. Start repairing the next table. Otherwise, use the following procedure:

  1. Make a backup of the data file before continuing.
  2. Use myisamchk -r tbl_name (-r means ``recovery mode''). This will remove incorrect records and deleted records from the data file and reconstruct the index file.
  3. If the preceding step fails, use myisamchk --safe-recover tbl_name. Safe recovery mode uses an old recovery method that handles a few cases that regular recovery mode doesn't (but is slower).

If you get weird errors when repairing (such as out of memory errors), or if myisamchk crashes, go to Stage 3.

Stage 3: Difficult repair

You should reach this stage only if the first 16KB block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it's necessary to create a new index file. Do so as follows:

  1. Move the data file to some safe place.
  2. Use the table description file to create new (empty) data and index files:
    shell> mysql db_name
    mysql> SET AUTOCOMMIT=1;
    mysql> TRUNCATE TABLE tbl_name;
    mysql> quit
    
    If your version of MySQL doesn't have TRUNCATE TABLE, use DELETE FROM tbl_name instead.
  3. Copy the old data file back onto the newly created data file. (Don't just move the old file back onto the new file; you want to retain a copy in case something goes wrong.)

Go back to Stage 2. myisamchk -r -q should work now. (This shouldn't be an endless loop.)

As of MySQL 4.0.2, you can also use REPAIR TABLE tbl_name USE_FRM, which performs the whole procedure automatically.

Stage 4: Very difficult repair

You should reach this stage only if the `.frm' description file has also crashed. That should never happen, because the description file isn't changed after the table is created:

  1. Restore the description file from a backup and go back to Stage 3. You can also restore the index file and go back to Stage 2. In the latter case, you should start with myisamchk -r.
  2. If you don't have a backup but know exactly how the table was created, create a copy of the table in another database. Remove the new data file, then move the `.frm' description and `.MYI' index files from the other database to your crashed database. This gives you new description and index files, but leaves the `.MYD' data file alone. Go back to Stage 2 and attempt to reconstruct the index file.

5.7.2.10 Table Optimization

To coalesce fragmented records and eliminate wasted space resulting from deleting or updating records, run myisamchk in recovery mode:

shell> myisamchk -r tbl_name

You can optimize a table in the same way by using the SQL OPTIMIZE TABLE statement. OPTIMIZE TABLE does a repair of the table and a key analysis, and also sorts the index tree to give faster key lookups. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use OPTIMIZE TABLE. See section 13.5.2.5 OPTIMIZE TABLE Syntax.

myisamchk also has a number of other options you can use to improve the performance of a table:

For a full description of the options, see section 5.7.2.1 myisamchk Invocation Syntax.

5.7.3 Setting Up a Table Maintenance Schedule

It is a good idea to perform table checks on a regular basis rather than waiting for problems to occur. One way to check and repair MyISAM tables is with the CHECK TABLE and REPAIR TABLE statements. These are available starting with MySQL 3.23.16. See section 13.5.2.3 CHECK TABLE Syntax and section 13.5.2.6 REPAIR TABLE Syntax.

Another way to check tables is to use myisamchk. For maintenance purposes, you can use myisamchk -s. The -s option (short for --silent) causes myisamchk to run in silent mode, printing messages only when errors occur.

It's also a good idea to check tables when the server starts. For example, whenever the machine has done a restart in the middle of an update, you usually need to check all the tables that could have been affected. (These are ``expected crashed tables.'') To check MyISAM tables automatically, start the server with the --myisam-recover option, available as of MySQL 3.23.25. If your server is too old to support this option, you could add a test to mysqld_safe that runs myisamchk to check all tables that have been modified during the last 24 hours if there is an old `.pid' (process ID) file left after a restart. (The `.pid' file is created by mysqld when it starts and removed when it terminates normally. The presence of a `.pid' file at system startup time indicates that mysqld terminated abnormally.)

An even better test would be to check any table whose last-modified time is more recent than that of the `.pid' file.

You should also check your tables regularly during normal system operation. At MySQL AB, we run a cron job to check all our important tables once a week, using a line like this in a `crontab' file:

35 0 * * 0 /path/to/myisamchk --fast --silent /path/to/datadir/*/*.MYI

This prints out information about crashed tables so that we can examine and repair them when needed.

Because we haven't had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough for us.

We recommend that to start with, you execute myisamchk -s each night on all tables that have been updated during the last 24 hours, until you come to trust MySQL as much as we do.

Normally, MySQL tables need little maintenance. If you are changing MyISAM tables with dynamic-sized rows (tables with VARCHAR, BLOB, or TEXT columns) or have tables with many deleted rows you may want to defragment/reclaim space from the tables from time to time (once a month?).

You can do this by using OPTIMIZE TABLE on the tables in question. Or, if you can stop the mysqld server for a while, change location into the data directory and use this command while the server is stopped:

shell> myisamchk -r -s --sort-index -O sort_buffer_size=16M */*.MYI

For ISAM tables, the command is similar:

shell> isamchk -r -s --sort-index -O sort_buffer_size=16M */*.ISM

5.7.4 Getting Information About a Table

To obtain a description of a table or statistics about it, use the commands shown here. We explain some of the information in more detail later:

Sample output for some of these commands follows. They are based on a table with these data and index file sizes:

-rw-rw-r--   1 monty    tcx     317235748 Jan 12 17:30 company.MYD
-rw-rw-r--   1 davida   tcx      96482304 Jan 12 18:35 company.MYM

Example of myisamchk -d output:

MyISAM file:     company.MYI
Record format:   Fixed length
Data records:    1403698  Deleted blocks:         0
Recordlength:    226

table description:
Key Start Len Index   Type
1   2     8   unique  double
2   15    10  multip. text packed stripped
3   219   8   multip. double
4   63    10  multip. text packed stripped
5   167   2   multip. unsigned short
6   177   4   multip. unsigned long
7   155   4   multip. text
8   138   4   multip. unsigned long
9   177   4   multip. unsigned long
    193   1           text

Example of myisamchk -d -v output:

MyISAM file:         company
Record format:       Fixed length
File-version:        1
Creation time:       1999-10-30 12:12:51
Recover time:        1999-10-31 19:13:01
Status:              checked
Data records:            1403698  Deleted blocks:              0
Datafile parts:          1403698  Deleted data:                0
Datafile pointer (bytes):      3  Keyfile pointer (bytes):     3
Max datafile length:  3791650815  Max keyfile length: 4294967294
Recordlength:                226

table description:
Key Start Len Index   Type                  Rec/key     Root Blocksize
1   2     8   unique  double                      1 15845376      1024
2   15    10  multip. text packed stripped        2 25062400      1024
3   219   8   multip. double                     73 40907776      1024
4   63    10  multip. text packed stripped        5 48097280      1024
5   167   2   multip. unsigned short           4840 55200768      1024
6   177   4   multip. unsigned long            1346 65145856      1024
7   155   4   multip. text                     4995 75090944      1024
8   138   4   multip. unsigned long              87 85036032      1024
9   177   4   multip. unsigned long             178 96481280      1024
    193   1           text

Example of myisamchk -eis output:

Checking MyISAM file: company
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:  98%  Packed:   17%

Records:          1403698    M.recordlength:     226
Packed:             0%
Recordspace used:     100%   Empty space:          0%
Blocks/Record:   1.00
Record blocks:    1403698    Delete blocks:        0
Recorddata:     317235748    Deleted data:         0
Lost space:             0    Linkdata:             0

User time 1626.51, System time 232.36
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 627, Swaps 0
Blocks in 0 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 639, Involuntary context switches 28966

Example of myisamchk -eiv output:

Checking MyISAM file: company
Data records: 1403698   Deleted blocks:       0
- check file-size
- check delete-chain
block_size 1024:
index  1:
index  2:
index  3:
index  4:
index  5:
index  6:
index  7:
index  8:
index  9:
No recordlinks
- check index reference
- check data record references index: 1
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 2
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
- check data record references index: 3
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
- check data record references index: 5
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 6
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 7
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 8
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 9
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:   9%  Packed:   17%

- check records and index references
[LOTS OF ROW NUMBERS DELETED]

Records:         1403698   M.recordlength:   226   Packed:           0%
Recordspace used:    100%  Empty space:        0%  Blocks/Record: 1.00
Record blocks:   1403698   Delete blocks:      0
Recorddata:    317235748   Deleted data:       0
Lost space:            0   Linkdata:           0

User time 1639.63, System time 251.61
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0
Blocks in 4 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 10604, Involuntary context switches 122798

Explanations for the types of information myisamchk produces are given here. ``Keyfile'' refers to the index file. ``Record'' and ``row'' are synonymous.

If a table has been compressed with myisampack, myisamchk -d prints additional information about each table column. See section 8.2 myisampack, the MySQL Compressed Read-only Table Generator, for an example of this information and a description of what it means.

5.8 MySQL Localization and International Usage

This section describes how to configure the server to use different character sets. It also discusses how to set the server's time zone and enable per-connection time zone support.

5.8.1 The Character Set Used for Data and Sorting

By default, MySQL uses the ISO-8859-1 (Latin1) character set with sorting according to Swedish/Finnish rules. These defaults are suitable for the United States and most of western Europe.

All MySQL binary distributions are compiled with --with-extra-charsets=complex. This adds code to all standard programs that enables them to handle latin1 and all multi-byte character sets within the binary. Other character sets will be loaded from a character-set definition file when needed.

The character set determines what characters are allowed in names. It also determines how strings are sorted by the ORDER BY and GROUP BY clauses of the SELECT statement.

You can change the character set with the --default-character-set option when you start the server. The character sets available depend on the --with-charset=charset and --with-extra-charsets= list-of-charsets | complex | all | none options to configure, and the character set configuration files listed in `SHAREDIR/charsets/Index'. See section 2.8.2 Typical configure Options.

As of MySQL 4.1.1, you can also change the character set collation with the --default-collation option when you start the server. The collation must be a legal collation for the default character set. (Use the SHOW COLLATION statement to determine which collations are available for each character set.) See section 2.8.2 Typical configure Options.

If you change the character set when running MySQL, that may also change the sort order. Consequently, you must run myisamchk -r -q --set-character-set=charset on all tables, or your indexes may not be ordered correctly.

When a client connects to a MySQL server, the server indicates to the client what the server's default character set is. The client will switch to use this character set for this connection.

You should use mysql_real_escape_string() when escaping strings for an SQL query. mysql_real_escape_string() is identical to the old mysql_escape_string() function, except that it takes the MYSQL connection handle as the first parameter so that the appropriate character set can be taken into account when escaping characters.

If the client is compiled with different paths than where the server is installed and the user who configured MySQL didn't include all character sets in the MySQL binary, you must tell the client where it can find the additional character sets it will need if the server runs with a different character set than the client.

You can do this by specifying a --character-sets-dir option to indicate the path to the directory in which the dynamic MySQL character sets are stored. For example, you can put the following in an option file:

[client]
character-sets-dir=/usr/local/mysql/share/mysql/charsets

You can force the client to use specific character set as follows:

[client]
default-character-set=charset

This is normally unnecessary, however.

5.8.1.1 Using the German Character Set

In MySQL 4.0, to get German sorting order, you should start mysqld with a --default-character-set=latin1_de option. This affects server behavior in several ways:

In MySQL 4.1 and up, character set and collation are specified separately. You should select the latin1 character set and either the latin1_german1_ci or latin1_german2_ci collation. For example, to start the server with the latin1_german1_ci collation, use the --character-set-server=latin1 and --collation-server=latin1_german1_ci options.

For information on the differences between these two collations, see section 10.11.2 West European Character Sets.

5.8.2 Setting the Error Message Language

By default, mysqld produces error messages in English, but they can also be displayed in any of these other languages: Czech, Danish, Dutch, Estonian, French, German, Greek, Hungarian, Italian, Japanese, Korean, Norwegian, Norwegian-ny, Polish, Portuguese, Romanian, Russian, Slovak, Spanish, or Swedish.

To start mysqld with a particular language for error messages, use the --language or -L option. The option value can be a language name or the full path to the error message file. For example:

shell> mysqld --language=swedish

Or:

shell> mysqld --language=/usr/local/share/swedish

The language name should be specified in lowercase.

The language files are located (by default) in the `share/LANGUAGE' directory under the MySQL base directory.

To change the error message file, you should edit the `errmsg.txt' file, and then execute the following command to generate the `errmsg.sys' file:

shell> comp_err errmsg.txt errmsg.sys

If you upgrade to a newer version of MySQL, remember to repeat your changes with the new `errmsg.txt' file.

5.8.3 Adding a New Character Set

This section discusses the procedure for adding add another character set to MySQL. You must have a MySQL source distribution to use these instructions.

To choose the proper procedure, decide whether the character set is simple or complex:

For example, latin1 and danish are simple character sets, whereas big5 and czech are complex character sets.

In the following procedures, the name of your character set is represented by MYSET.

For a simple character set, do the following:

  1. Add MYSET to the end of the `sql/share/charsets/Index' file. Assign a unique number to it.
  2. Create the file `sql/share/charsets/MYSET.conf'. (You can use a copy of `sql/share/charsets/latin1.conf' as the basis for this file.) The syntax for the file is very simple: See section 5.8.4 The Character Definition Arrays.
  3. Add the character set name to the CHARSETS_AVAILABLE and COMPILED_CHARSETS lists in configure.in.
  4. Reconfigure, recompile, and test.

For a complex character set, do the following:

  1. Create the file `strings/ctype-MYSET.c' in the MySQL source distribution.
  2. Add MYSET to the end of the `sql/share/charsets/Index' file. Assign a unique number to it.
  3. Look at one of the existing `ctype-*.c' files (such as `strings/ctype-big5.c') to see what needs to be defined. Note that the arrays in your file must have names like ctype_MYSET, to_lower_MYSET, and so on. These correspond to the arrays for a simple character set. See section 5.8.4 The Character Definition Arrays.
  4. Near the top of the file, place a special comment like this:
    /*
     * This comment is parsed by configure to create ctype.c,
     * so don't change it unless you know what you are doing.
     *
     * .configure. number_MYSET=MYNUMBER
     * .configure. strxfrm_multiply_MYSET=N
     * .configure. mbmaxlen_MYSET=N
     */
    
    The configure program uses this comment to include the character set into the MySQL library automatically. The strxfrm_multiply and mbmaxlen lines are explained in the following sections. You need include them only if you need the string collating functions or the multi-byte character set functions, respectively.
  5. You should then create some of the following functions: See section 5.8.5 String Collating Support.
  6. Add the character set name to the CHARSETS_AVAILABLE and COMPILED_CHARSETS lists in configure.in.
  7. Reconfigure, recompile, and test.

The `sql/share/charsets/README' file includes additional instructions.

If you want to have the character set included in the MySQL distribution, mail a patch to the MySQL internals mailing list. See section 1.4.1.1 The MySQL Mailing Lists.

5.8.4 The Character Definition Arrays

to_lower[] and to_upper[] are simple arrays that hold the lowercase and uppercase characters corresponding to each member of the character set. For example:

to_lower['A'] should contain 'a'
to_upper['a'] should contain 'A'

sort_order[] is a map indicating how characters should be ordered for comparison and sorting purposes. Quite often (but not for all character sets) this is the same as to_upper[], which means that sorting will be case-insensitive. MySQL will sort characters based on the values of sort_order[] elements. For more complicated sorting rules, see the discussion of string collating in section 5.8.5 String Collating Support.

ctype[] is an array of bit values, with one element for one character. (Note that to_lower[], to_upper[], and sort_order[] are indexed by character value, but ctype[] is indexed by character value + 1. This is an old legacy convention to be able to handle EOF.)

You can find the following bitmask definitions in `m_ctype.h':

#define _U      01      /* Uppercase */
#define _L      02      /* Lowercase */
#define _N      04      /* Numeral (digit) */
#define _S      010     /* Spacing character */
#define _P      020     /* Punctuation */
#define _C      040     /* Control character */
#define _B      0100    /* Blank */
#define _X      0200    /* heXadecimal digit */

The ctype[] entry for each character should be the union of the applicable bitmask values that describe the character. For example, 'A' is an uppercase character (_U) as well as a hexadecimal digit (_X), so ctype['A'+1] should contain the value:

_U + _X = 01 + 0200 = 0201

5.8.5 String Collating Support

If the sorting rules for your language are too complex to be handled with the simple sort_order[] table, you need to use the string collating functions.

Right now the best documentation for this is the character sets that are already implemented. Look at the big5, czech, gbk, sjis, and tis160 character sets for examples.

You must specify the strxfrm_multiply_MYSET=N value in the special comment at the top of the file. N should be set to the maximum ratio the strings may grow during my_strxfrm_MYSET (it must be a positive integer).

5.8.6 Multi-Byte Character Support

If you want to add support for a new character set that includes multi-byte characters, you need to use the multi-byte character functions.

Right now the best documentation on this consists of the character sets that are already implemented. Look at the euc_kr, gb2312, gbk, sjis, and ujis character sets for examples. These are implemented in the `ctype-charset.c' files in the `strings' directory.

You must specify the mbmaxlen_MYSET=N value in the special comment at the top of the source file. N should be set to the size in bytes of the largest character in the set.

5.8.7 Problems With Character Sets

If you try to use a character set that is not compiled into your binary, you might run into the following problems:

For MyISAM tables, you can check the character set name and number for a table with myisamchk -dvv tbl_name.

5.8.8 MySQL Server Time Zone Support

Before MySQL 4.1.3, you can set the time zone for the server with the --timezone=timezone_name option to mysqld_safe. You can also set it by setting the TZ environment variable before you start mysqld.

The allowable values for --timezone or TZ are system-dependent. Consult your operating system documentation to see what values are acceptable.

Beginning with MySQL 4.1.3, the server maintains several time zone settings:

The current values of the global and per-connection time zones can be retrieved like this:

mysql> SELECT @@global.time_zone, @@session.time_zone;

timezone values can be given as strings indicating an offset from UTC, such as '+10:00' or '-6:00'. If the time zone-related tables in the mysql database have been created and populated, you can also used named time zones, such as 'Europe/Helsinki', 'US/Eastern', or 'MET'. The value 'SYSTEM' indicates that the time zone should be the same as the system time zone. Time zone names are not case sensitive.

The MySQL installation procedure creates the time zone tables in the mysql database, but does not load them. You must do so manually. (If you are upgrading to MySQL 4.1.3 or later from an earlier version, you should create the tables by upgrading your mysql database. Use the instructions in section 2.10.7 Upgrading the Grant Tables.)

If your system has its own zoneinfo database (the set of files describing time zones), you should use the mysql_tzinfo_to_sql program for filling the time zone tables. Examples of such systems are Linux, FreeBSD, Sun Solaris, and Mac OS X. One likely location for these files is the `/usr/share/zoneinfo' directory. If your system does not have a zoneinfo database, you can use the downloadable package described later in this section.

The mysql_tzinfo_to_sql program is used to load the time zone tables. On the command line, pass the zoneinfo directory pathname to mysql_tzinfo_to_sql and send the output into the mysql program. For example:

shell> mysql_tzinfo_to_sql /usr/share/zoneinfo | mysql -u root mysql

mysql_tzinfo_to_sql reads your system's time zone files and generates SQL statements from them. mysql processes those statements to load the time zone tables.

mysql_tzinfo_to_sql also can be used to load a single time zone file, and to generate leap second information.

To load a single time zone file tz_file that corresponds to a time zone name tz_name, invoke mysql_tzinfo_to_sql like this:

shell> mysql_tzinfo_to_sql tz_file tz_name | mysql -u root mysql

If your time zone needs to account for leap seconds, initialize the leap second information like this, where tz_file is the name of your time zone file:

shell> mysql_tzinfo_to_sql --leap tz_file | mysql -u root mysql

If your system doesn't have a zoneinfo database (for example, Windows or HP-UX), you can use the package of pre-built time zone tables that is available for download at http://dev.mysql.com/downloads/timezones.html. This package contains `.frm', `.MYD', and `.MYI' files for the MyISAM time zone tables. These tables should belong to the mysql database, so you should place the files in the `mysql' subdirectory of your MySQL server's data directory. The server should be shut down while you do this.

Warning! Please don't use the downloadable package if your system has a zoneinfo database. Use the mysql_tzinfo_to_sql utility instead! Otherwise, you may cause a difference in datetime handling between MySQL and other applications on your system.

5.9 The MySQL Log Files

MySQL has several different log files that can help you find out what's going on inside mysqld:

Log File Types of Information Logged to File
The error log Logs problems encountered starting, running, or stopping mysqld.
The isam log Logs all changes to the ISAM tables. Used only for debugging the isam code.
The query log Logs established client connections and executed statements.
The update log Logs statements that change data. This log is deprecated.
The binary log Logs all statements that change data. Also used for replication.
The slow log Logs all queries that took more than long_query_time seconds to execute or didn't use indexes.

By default, all logs are created in the mysqld data directory. You can force mysqld to close and reopen the log files (or in some cases switch to a new log) by flushing the logs. Log flushing occurs when you issue a FLUSH LOGS statement or execute mysqladmin flush-logs or mysqladmin refresh. See section 13.5.5.2 FLUSH Syntax.

If you are using MySQL replication capabilities, slave replication servers maintain additional log files called relay logs. These are discussed in section 6 Replication in MySQL.

5.9.1 The Error Log

The error log file contains information indicating when mysqld was started and stopped and also any critical errors that occur while the server is running.

If mysqld dies unexpectedly and mysqld_safe needs to restart it, mysqld_safe will write a restarted mysqld message to the error log. If mysqld notices a table that needs to be automatically checked or repaired, it writes a message to the error log.

On some operating systems, the error log will contain a stack trace if mysqld dies. The trace can be used to determine where mysqld died. See section E.1.4 Using a Stack Trace.

Beginning with MySQL 4.0.10, you can specify where mysqld stores the error log file with the --log-error[=file_name] option. If no file_name value is given, mysqld uses the name `host_name.err' and writes the file in the data directory. (Prior to MySQL 4.0.10, the Windows error log name is `mysql.err'.) If you execute FLUSH LOGS, the error log is renamed with a suffix of -old and mysqld creates a new empty log file.

In older MySQL versions on Unix, error log handling was done by mysqld_safe which redirected the error file to host_name.err. You could change this filename by specifying a --err-log=file_name option to mysqld_safe.

If you don't specify --log-error, or (on Windows) if you use the --console option, errors are written to stderr, the standard error output. Usually this is your terminal.

On Windows, error output is always written to the .err file if --console is not given.

5.9.2 The General Query Log

If you want to know what happens within mysqld, you should start it with the --log[=file_name] or -l [file_name] option. If no file_name value is given, the default name is `host_name.log' This will log all connections and statements to the log file. This log can be very useful when you suspect an error in a client and want to know exactly what the client sent to mysqld.

Older versions of the mysql.server script (from MySQL 3.23.4 to 3.23.8) pass a --log option to safe_mysqld to enable the general query log. If you need better performance when you start using MySQL in a production environment, you can remove the --log option from mysql.server or change it to --log-bin. See section 5.9.4 The Binary Log.

mysqld writes statements to the query log in the order that it receives them. This may be different from the order in which they are executed. This is in contrast to the update log and the binary log, which are written after the query is executed, but before any locks are released. (The query log also contains all statements, whereas the update and binary logs do not contain statements that only select data.)

Server restarts and log flushing do not cause a new general query log file to be generated (although flushing closes and reopens it). On Unix, you can rename the file and create a new one by using the following commands:

shell> mv hostname.log hostname-old.log
shell> mysqladmin flush-logs
shell> cp hostname-old.log to-backup-directory
shell> rm hostname-old.log

On Windows, you cannot rename the log file while the server has it open. You must stop the server and rename the log. Then restart the server to create a new log.

5.9.3 The Update Log

Note: The update log has been deprecated and replaced by the binary log. See section 5.9.4 The Binary Log. The binary log can do anything the old update log could do, and more. The update log is unavailable as of MySQL 5.0.0.

When started with the --log-update[=file_name] option, mysqld writes a log file containing all SQL statements that update data. If no file_name value is given, the default name is name of the host machine. If a filename is given, but it doesn't contain a leading path, the file is written in the data directory. If `file_name' doesn't have an extension, mysqld creates log files with names of the form file_name.###, where ### is a number that is incremented each time you start the server or flush the logs.

Note: For this naming scheme to work, you must not create your own files with the same names as those that might be used for the log file sequence.

Update logging is smart because it logs only statements that really update data. So, an UPDATE or a DELETE with a WHERE that finds no rows is not written to the log. It even skips UPDATE statements that set a column to the value it already has.

The update logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that statements are logged in execution order.

If you want to update a database from update log files, you could do the following (assuming that your update logs have names of the form `file_name.###'):

shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql

ls is used to sort the update log filenames into the right order.

This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.

5.9.4 The Binary Log

The binary log has replaced the old update log, which is unavailable starting from MySQL 5.0. The binary log contains all information that is available in the update log in a more efficient format and in a manner that is transactionally safe.

The binary log contains all statements which updated data or (starting from MySQL 4.1.3) could potentially have updated it (for example, a DELETE which matched no rows).

The binary log also contains information about how long each statement took that updated the database. It doesn't contain statements that don't modify any data. If you want to log all statements (for example, to identify a problem query) you should use the general query log. See section 5.9.2 The General Query Log.

The primary purpose of the binary log is to be able to update the database during a restore operation as fully as possible, because the binary log will contain all updates done after a backup was made.

The binary log is also used on master replication servers as a record of the statements to be sent to slave servers. See section 6 Replication in MySQL.

Running the server with the binary log enabled makes performance about 1% slower. However, the benefits of the binary log for restore operations and in allowing you to set up replication generally outweigh this minor performance decrement.

When started with the --log-bin[=file_name] option, mysqld writes a log file containing all SQL commands that update data. If no file_name value is given, the default name is the name of the host machine followed by -bin. If file name is given, but it doesn't contain a path, the file is written in the data directory.

If you supply an extension in the log name (for example, --log-bin=file_name.extension), the extension is silently removed and ignored.

mysqld appends a numeric extension to the binary log name. The number is incremented each time you start the server or flush the logs. A new binary log also is created automatically when the current log's size reaches max_binlog_size. A binary log may become larger than max_binlog_size if you are using large transactions: A transaction is written to the binary log in one piece, never split between binary logs.

To be able to know which different binary log files have been used, mysqld also creates a binary log index file that contains the name of all used binary log files. By default this has the same name as the binary log file, with the extension '.index'. You can change the name of the binary log index file with the --log-bin-index[=file_name] option. You should not manually edit this file while mysqld is running; doing so would confuse mysqld.

You can delete all binary log files with the RESET MASTER statement, or only some of them with PURGE MASTER LOGS. See section 13.5.5.5 RESET Syntax and section 13.6.1 SQL Statements for Controlling Master Servers.

The binary log format has some known limitations which can affect recovery from backups, especially in old versions. These caveats which also affect replication are listed at section 6.7 Replication Features and Known Problems. One caveat which does not affect replication but only recovery with mysqlbinlog: before MySQL 4.1, mysqlbinlog could not prepare output suitable for mysql if the binary log contained interlaced statements originating from different clients that used temporary tables of the same name. This is fixed in MySQL 4.1. However, the problem still existed for LOAD DATA INFILE statements until it was fixed in MySQL 4.1.8.

You can use the following options to mysqld to affect what is logged to the binary log. See also the discussion that follows this option list.

--binlog-do-db=db_name
Tells the master that it should log updates to the binary log if the current database (that is, the one selected by USE) is db_name. All other databases that are not explicitly mentioned are ignored. If you use this, you should ensure that you only do updates in the current database. Observe that there is an exception to the CREATE/ALTER/DROP DATABASE statements, which use the database manipulated to decide if it should log the statement rather than the current database. An example of what does not work as you might expect: If the server is started with binlog-do-db=sales, and you do USE prices; UPDATE sales.january SET amount=amount+1000;, this statement will not be written into the binary log.
--binlog-ignore-db=db_name
Tells the master that updates where the current database (that is, the one selected by USE) is db_name should not be stored in the binary log. If you use this, you should ensure that you only do updates in the current database. An example of what does not work as you might expect: If the server is started with binlog-ignore-db=sales, and you do USE prices; UPDATE sales.january SET amount=amount+1000;, this statement will be written into the binary log. Similar to the case for --binlog-do-db, there is an exception to the CREATE/ALTER/DROP DATABASE statements, which use the database manipulated to decide if it should log the statement rather than the current database.

To log or ignore multiple databases, specify the appropriate option multiple times, once for each database.

The rules for logging or ignoring updates to the binary log are evaluated according to the following rules. Observe that there is an exception for CREATE/ALTER/DROP DATABASE statements. In those cases, the database being created/altered/dropped replace the current database in the rules below.

  1. Are there binlog-do-db or binlog-ignore-db rules?
  2. There are some rules (binlog-do-db or binlog-ignore-db or both). Is there a current database (has any database been selected by USE?)?
  3. There is a current database. Are there some binlog-do-db rules?
  4. There are some binlog-ignore-db rules. Does the current database match any of the binlog-ignore-db rules?

For example, a slave running with only binlog-do-db=sales will not write to the binary log any statement whose current database is different from sales (in other words, binlog-do-db can sometimes mean ``ignore other databases'').

If you are using replication, you should not delete old binary log files until you are sure that no slave still needs to use them. One way to do this is to do mysqladmin flush-logs once a day and then remove any logs that are more than three days old. You can remove them manually, or preferably using PURGE MASTER LOGS (see section 13.6.1 SQL Statements for Controlling Master Servers), which will also safely update the binary log index file for you (and which can take a date argument since MySQL 4.1)

A client with the SUPER privilege can disable binary logging of its own statements by using a SET SQL_LOG_BIN=0 statement. See section 13.5.3 SET Syntax.

You can examine the binary log file with the mysqlbinlog utility. This can be useful when you want to reprocess statements in the log. For example, you can update a MySQL server from the binary log as follows:

shell> mysqlbinlog log-file | mysql -h server_name

See section 8.5 The mysqlbinlog Binary Log Utility for more information on the mysqlbinlog utility and how to use it.

If you are using transactions, you must use the MySQL binary log for backups instead of the old update log.

The binary logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order.

Updates to non-transactional tables are stored in the binary log immediately after execution. For transactional tables such as BDB or InnoDB tables, all updates (UPDATE, DELETE, or INSERT) that change tables are cached until a COMMIT statement is received by the server. At that point, mysqld writes the whole transaction to the binary log before the COMMIT is executed. When the thread that handles the transaction starts, it allocates a buffer of binlog_cache_size to buffer queries. If a statement is bigger than this, the thread opens a temporary file to store the transaction. The temporary file is deleted when the thread ends.

The Binlog_cache_use status variable shows the number of transactions that used this buffer (and possibly a temporary file) for storing statements. The Binlog_cache_disk_use status variable shows how many of those transactions actually did have to use a temporary file. These two variables can be used for tuning binlog_cache_size to a large enough value that avoids the use of temporary files.

The max_binlog_cache_size (default 4GB) can be used to restrict the total size used to cache a multiple-statement transaction. If a transaction is larger than this, it will fail and roll back.

If you are using the update log or binary log, concurrent inserts will be converted to normal inserts when using CREATE ... SELECT or INSERT ... SELECT. This is to ensure that you can re-create an exact copy of your tables by applying the log on a backup.

The binary log format is different in versions 3.23, 4.0, and 5.0.0. Those format changes were required to implement enhancements to replication. MySQL 4.1 has the same binary log format as 4.0. See section 6.5 Replication Compatibility Between MySQL Versions.

By default, the binary log is not synchronized to disk at each write. So if the operating system or machine (not only the MySQL server) crashes there is a chance that the last statements of the binary log are lost. To prevent this, you can make the binary log be synchronized to disk after every Nth binary log write, with the sync_binlog global variable (1 being the safest value, but also the slowest). See section 5.2.3 Server System Variables. Even with this set to 1, there is still the chance of an inconsistency between the tables content and the binary log content in case of crash. For example, if using InnoDB tables, and the MySQL server processes a COMMIT statement, it writes the whole transaction to the binary log and then commits this transaction into InnoDB. If it crashes between those two operations, at restart the transaction will be rolled back by InnoDB but still exist in the binary log. This problem can be solved with the --innodb-safe-binlog option (available starting from MySQL 4.1.3), which adds consistency between the content of InnoDB tables and the binary log. For this option to really bring safety to you, the MySQL server should also be configured to synchronize to disk, at every transaction, the binary log (sync_binlog=1) and (which is true by default) the InnoDB logs. The effect of this option is that at restart after a crash, after doing a rollback of transactions, the MySQL server will cut rolled back InnoDB transactions from the binary log. This ensures that the binary log reflects the exact data of InnoDB tables, and so, that the slave remains in sync with the master (not receiving a statement which has been rolled back). Note that --innodb-safe-binlog can be used even if the MySQL server updates other storage engines than InnoDB. Only statements/transactions affecting InnoDB tables are subject to being removed from the binary log at InnoDB's crash recovery. If at crash recovery the MySQL server discovers that the binary log is shorter than it should have been (that is, it lacks at least one successfully committed InnoDB transaction), which should not happen if sync_binlog=1 and the disk/filesystem do an actual sync when they are requested to (some don't), it will print an error message ("The binary log <name> is shorter than its expected size"). In this case, this binary log is not correct, replication should be restarted from a fresh master's data snapshot.

Before MySQL 4.1.9, a write to a binary log file or binary log index file that failed due to a full disk or an exceeded quota resulted in corruption of the file. Starting from MySQL 4.1.9, writes to the binary log file and binary log index file are handled the same way as writes to MyISAM tables. See section A.4.3 How MySQL Handles a Full Disk.

5.9.5 The Slow Query Log

When started with the --log-slow-queries[=file_name] option, mysqld writes a log file containing all SQL statements that took more than long_query_time seconds to execute. The time to acquire the initial table locks are not counted as execution time.

If no file_name value is given, the default is the name of the host machine with a suffix of -slow.log. If a filename is given, but doesn't contain a path, the file is written in the data directory.

A statement is logged to the slow query log after it has been executed and after all locks have been released. Log order may be different from execution order.

The slow query log can be used to find queries that take a long time to execute and are therefore candidates for optimization. However, examining a long slow query log can become a difficult task. To make this easier, you can pipe the slow query log through the mysqldumpslow command to get a summary of the queries that appear in the log.

If you also use the --log-long-format when logging slow queries, then queries that are not using indexes are logged as well. See section 5.2.1 mysqld Command-Line Options.

5.9.6 Log File Maintenance

The MySQL Server can create a number of different log files that make it easy to see what is going on. See section 5.9 The MySQL Log Files. However, you must clean up these files regularly to ensure that the logs don't take up too much disk space.

When using MySQL with logging enabled, you will want to back up and remove old log files from time to time and tell MySQL to start logging to new files. See section 5.7.1 Database Backups.

On a Linux (Red Hat) installation, you can use the mysql-log-rotate script for this. If you installed MySQL from an RPM distribution, the script should have been installed automatically. You should be careful with this script if you are using the binary log for replication! (You should not remove binary logs until you are certain that their contents have been processed by all slaves.)

On other systems, you must install a short script yourself that you start from cron to handle log files.

You can force MySQL to start using new log files by using mysqladmin flush-logs or by using the SQL statement FLUSH LOGS. If you are using MySQL 3.21, you must use mysqladmin refresh.

A log flushing operation does the following:

If you are using only an update log, you only have to rename the log file and then flush the logs before making a backup. For example, you can do something like this:

shell> cd mysql-data-directory
shell> mv mysql.log mysql.old
shell> mysqladmin flush-logs

Then make a backup and remove `mysql.old'.

5.10 Running Multiple MySQL Servers on the Same Machine

In some cases, you might want to run multiple mysqld servers on the same machine. You might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you may want to give different users access to different mysqld servers that they manage themselves. (For example, you might be an Internet Service Provider that wants to provide independent MySQL installations for different customers.)

To run multiple servers on a single machine, each server must have unique values for several operating parameters. These can be set on the command line or in option files. See section 4.3 Specifying Program Options.

At least the following options must be different for each server:

--port=port_num
--port controls the port number for TCP/IP connections.
--socket=path
--socket controls the Unix socket file path on Unix and the name of the named pipe on Windows. On Windows, it's necessary to specify distinct pipe names only for those servers that support named pipe connections.
--shared-memory-base-name=name
This option currently is used only on Windows. It designates the shared memory name used by a Windows server to allow clients to connect via shared memory. This option is new in MySQL 4.1.
--pid-file=path
This option is used only on Unix. It indicates the name of the file in which the server writes its process ID.

If you use the following log file options, they must be different for each server:

Log file options are described in section 5.9.6 Log File Maintenance.

If you want more performance, you can also specify the following options differently for each server, to spread the load between several physical disks:

Having different temporary directories is also recommended, to make it easier to determine which MySQL server created any given temporary file.

Generally, each server should also use a different data directory, which is specified using the --datadir=path option.

Warning: Normally you should never have two servers that update data in the same databases! This may lead to unpleasant surprises if your operating system doesn't support fault-free system locking! If (despite this warning) you run multiple servers using the same data directory and they have logging enabled, you must use the appropriate options to specify log filenames that are unique to each server. Otherwise, the servers will try to log to the same files. Please note that this kind of setup will only work with ISAM, MyISAM and MERGE tables, and not with any of the other storage engines.

The warning against sharing a data directory among servers also applies in an NFS environment. Allowing multiple MySQL servers to access a common data directory over NFS is a bad idea!

Make it easy for yourself: Forget about sharing a data directory among servers over NFS. A better solution is to have one computer that contains several CPUs and use an operating system that handles threads efficiently.

If you have multiple MySQL installations in different locations, normally you can specify the base installation directory for each server with the --basedir=path option to cause each server to use a different data directory, log files, and PID file. (The defaults for all these values are determined relative to the base directory). In that case, the only other options you need to specify are the --socket and --port options. For example, suppose that you install different versions of MySQL using `tar' file binary distributions. These will install in different locations, so you can start the server for each installation using the command bin/mysqld_safe under its corresponding base directory. mysqld_safe will determine the proper --basedir option to pass to mysqld, and you need specify only the --socket and --port options to mysqld_safe. (For versions of MySQL older than 4.0, use safe_mysqld rather than mysqld_safe.)

As discussed in the following sections, it is possible to start additional servers by setting environment variables or by specifying appropriate command-line options. However, if you need to run multiple servers on a more permanent basis, it will be more convenient to use option files to specify for each server those option values that must be unique to it.

5.10.1 Running Multiple Servers on Windows

You can run multiple servers on Windows by starting them manually from the command line, each with appropriate operating parameters. On Windows NT-based systems, you also have the option of installing several servers as Windows services and running them that way. General instructions for running MySQL servers from the command line or as services are given in section 2.3 Installing MySQL on Windows. This section describes how to make sure that you start each server with different values for those startup options that must be unique per server, such as the data directory. These options are described in section 5.10 Running Multiple MySQL Servers on the Same Machine.

5.10.1.1 Starting Multiple Windows Servers at the Command Line

To start multiple servers manually from the command line, you can specify the appropriate options on the command line or in an option file. It's more convenient to place the options in an option file, but it's necessary to make sure that each server gets its own set of options. To do this, create an option file for each server and tell the server the filename with a --defaults-file option when you run it.

Suppose that you want to run mysqld on port 3307 with a data directory of `C:\mydata1', and mysqld-max on port 3308 with a data directory of `C:\mydata2'. (To do this, make sure that before you start the servers, each data directory exists and has its own copy of the mysql database that contains the grant tables.)

Then create two option files. For example, create one file named `C:\my-opts1.cnf' that looks like this:

[mysqld]
datadir = C:/mydata1
port = 3307

Create a second file named `C:\my-opts2.cnf' that looks like this:

[mysqld]
datadir = C:/mydata2
port = 3308

Then start each server with its own option file:

C:\> C:\mysql\bin\mysqld --defaults-file=C:\my-opts1.cnf
C:\> C:\mysql\bin\mysqld-max --defaults-file=C:\my-opts2.cnf

On NT, each server will start in the foreground (no new prompt appears until the server exits later); you'll need to issue those two commands in separate console windows.

To shut down the servers, you must connect to the appropriate port number:

C:\> C:\mysql\bin\mysqladmin --port=3307 shutdown
C:\> C:\mysql\bin\mysqladmin --port=3308 shutdown

Servers configured as just described will allow clients to connect over TCP/IP. If your version of Windows supports named pipes and you also want to allow named pipe connections, use the mysqld-nt or mysqld-max-nt servers and specify options that enable the named pipe and specify its name. Each server that supports named pipe connections must use a unique pipe name. For example, the `C:\my-opts1.cnf' file might be written like this:

[mysqld]
datadir = C:/mydata1
port = 3307
enable-named-pipe
socket = mypipe1

Then start the server this way:

C:\> C:\mysql\bin\mysqld-nt --defaults-file=C:\my-opts1.cnf

Modify `C:\my-opts2.cnf' similarly for use by the second server.

5.10.1.2 Starting Multiple Windows Servers as Services

On NT-based systems, a MySQL server can be run as a Windows service. The procedures for installing, controlling, and removing a single MySQL service are described in section 2.3.12 Starting MySQL as a Windows Service.

As of MySQL 4.0.2, you can install multiple servers as services. In this case, you must make sure that each server uses a different service name in addition to all the other parameters that must be unique per server.

For the following instructions, assume that you want to run the mysqld-nt server from two different versions of MySQL that are installed at `C:\mysql-4.0.8' and `C:\mysql-4.0.17', respectively. (This might be the case if you're running 4.0.8 as your production server, but want to test 4.0.17 before upgrading to it.)

The following principles apply when installing a MySQL service with the --install or --install-manual option:

Note: Before MySQL 4.0.17, only a server installed using the default service name (MySQL) or one installed explicitly with a service name of mysqld will read the [mysqld] group in the standard option files. As of 4.0.17, all servers read the [mysqld] group if they read the standard option files, even if they are installed using another service name. This allows you to use the [mysqld] group for options that should be used by all MySQL services, and an option group named after each service for use by the server installed with that service name.

Based on the preceding information, you have several ways to set up multiple services. The following instructions describe some examples. Before trying any of them, be sure that you shut down and remove any existing MySQL services first.

To remove multiple services, use mysqld --remove for each one, specifying a service name following the --remove option. If the service name is the default (MySQL), you can omit it.

5.10.2 Running Multiple Servers on Unix

The easiest way is to run multiple servers on Unix is to compile them with different TCP/IP ports and Unix socket files so that each one is listening on different network interfaces. Also, by compiling in different base directories for each installation, that automatically results in different compiled-in data directory, log file, and PID file locations for each of your servers.

Assume that an existing server is configured for the default TCP/IP port number (3306) and Unix socket file (`/tmp/mysql.sock'). To configure a new server to have different operating parameters, use a configure command something like this:

shell> ./configure --with-tcp-port=port_number \
             --with-unix-socket-path=file_name \
             --prefix=/usr/local/mysql-4.0.17

Here, port_number and file_name must be different from the default TCP/IP port number and Unix socket file pathname, and the --prefix value should specify an installation directory different than the one under which the existing MySQL installation is located.

If you have a MySQL server listening on a given port number, you can use the following command to find out what operating parameters it is using for several important configurable variables, including the base directory and Unix socket filename:

shell> mysqladmin --host=host_name --port=port_number variables

With the information displayed by that command, you can tell what option values not to use when configuring an additional server.

Note that if you specify localhost as a hostname, mysqladmin will default to using a Unix socket file connection rather than TCP/IP. In MySQL 4.1, you can explicitly specify the connection protocol to use by using the --protocol={TCP | SOCKET | PIPE | MEMORY} option.

You don't have to compile a new MySQL server just to start with a different Unix socket file and TCP/IP port number. It is also possible to specify those values at runtime. One way to do so is by using command-line options:

shell> mysqld_safe --socket=file_name --port=port_number

To start a second server, provide different --socket and --port option values, and pass a --datadir=path option to mysqld_safe so that the server uses a different data directory.

Another way to achieve a similar effect is to use environment variables to set the Unix socket filename and TCP/IP port number:

shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
shell> MYSQL_TCP_PORT=3307
shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
shell> mysql_install_db --user=mysql
shell> mysqld_safe --datadir=/path/to/datadir &

This is a quick way of starting a second server to use for testing. The nice thing about this method is that the environment variable settings will apply to any client programs that you invoke from the same shell. Thus, connections for those clients automatically will be directed to the second server!

section F Environment Variables includes a list of other environment variables you can use to affect mysqld.

For automatic server execution, your startup script that is executed at boot time should execute the following command once for each server with an appropriate option file path for each command:

mysqld_safe --defaults-file=path

Each option file should contain option values specific to a given server.

On Unix, the mysqld_multi script is another way to start multiple servers. See section 5.1.5 The mysqld_multi Program for Managing Multiple MySQL Servers.

5.10.3 Using Client Programs in a Multiple-Server Environment

When you want to connect with a client program to a MySQL server that is listening to different network interfaces than those compiled into your client, you can use one of the following methods:

5.11 The MySQL Query Cache

From version 4.0.1 on, MySQL Server features a query cache. When in use, the query cache stores the text of a SELECT query together with the corresponding result that was sent to the client. If the identical query is received later, the server retrieves the results from the query cache rather than parsing and executing the query again.

The query cache is extremely useful in an environment where (some) tables don't change very often and you have a lot of identical queries. This is a typical situation for many Web servers that generate a lot of dynamic pages based on database content.

Note: The query cache does not return stale data. When tables are modified, any relevant entries in the query cache are flushed.

Note: The query cache does not work in an environment where you have many mysqld servers updating the same MyISAM tables.

Some performance data for the query cache follow. These results were generated by running the MySQL benchmark suite on a Linux Alpha 2 x 500MHz system with 2GB RAM and a 64MB query cache.

To disable the query cache at server startup, set the query_cache_size system variable to 0. By disabling the query cache code, there is no noticeable overhead. Query cache capabilities can be excluded from the server entirely by using the --without-query-cache option to configure when compiling MySQL.

5.11.1 How the Query Cache Operates

This section describes how the query cache works when it is operational. section 5.11.3 Query Cache Configuration describes how to control whether or not it is operational.

Queries are compared before parsing, so the following two queries are regarded as different by the query cache:

SELECT * FROM tbl_name
Select * from tbl_name

Queries must be exactly the same (byte for byte) to be seen as identical. In addition, query strings that are identical may be treated as different for other reasons. Queries that use different databases, different protocol versions, or different default character sets are considered different queries and are cached separately.

If a query result is returned from query cache, the server increments the Qcache_hits status variable, not Com_select. See section 5.11.4 Query Cache Status and Maintenance.

If a table changes, then all cached queries that use the table become invalid and are removed from the cache. This includes queries that use MERGE tables that map to the changed table. A table can be changed by many types of statements, such as INSERT, UPDATE, DELETE, TRUNCATE, ALTER TABLE, DROP TABLE, or DROP DATABASE.

Transactional InnoDB tables that have been changed are invalidated when a COMMIT is performed.

In MySQL 4.0, the query cache is disabled within transactions (it does not return results). Beginning with MySQL 4.1.1, the query cache also works within transactions when using InnoDB tables (it uses the table version number to detect whether or not its contents are still current).

Before MySQL 5.0, a query that begins with a leading comment might be cached, but could not be fetched from the cache. This problem is fixed in MySQL 5.0.

The query cache works for SELECT SQL_CALC_FOUND_ROWS ... and SELECT FOUND_ROWS() type queries. FOUND_ROWS() returns the correct value even if the preceding query was fetched from the cache because the number of found rows is also stored in the cache.

A query cannot be cached if it contains any of the following functions:

BENCHMARK() CONNECTION_ID() CURDATE()
CURRENT_DATE() CURRENT_TIME() CURRENT_TIMESTAMP()
CURTIME() DATABASE() ENCRYPT() with one parameter
FOUND_ROWS() GET_LOCK() LAST_INSERT_ID()
LOAD_FILE() MASTER_POS_WAIT() NOW()
RAND() RELEASE_LOCK() SYSDATE()
UNIX_TIMESTAMP() with no parameters USER()

A query also will not be cached under these conditions:

5.11.2 Query Cache SELECT Options

There are two query cache-related options that may be specified in a SELECT statement:

SQL_CACHE
The query result is cached if the value of the query_cache_type system variable is ON or DEMAND.
SQL_NO_CACHE
The query result is not cached.

Examples:

SELECT SQL_CACHE id, name FROM customer;
SELECT SQL_NO_CACHE id, name FROM customer;

5.11.3 Query Cache Configuration

The have_query_cache server system variable indicates whether the query cache is available:

mysql> SHOW VARIABLES LIKE 'have_query_cache';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| have_query_cache | YES   |
+------------------+-------+

Several other system variables control query cache operation. These can be set in an option file or on the command line when starting mysqld. The query cache-related system variables all have names that begin with query_cache_. They are described briefly in section 5.2.3 Server System Variables, with additional configuration information given here.

To set the size of the query cache, set the query_cache_size system variable. Setting it to 0 disables the query cache. The default cache size is 0; that is, the query cache is disabled.

If the query cache is enabled, the query_cache_type variable influences how it works. This variable can be set to the following values:

Setting the GLOBAL value of query_cache_type determines query cache behavior for all clients that connect after the change is made. Individual clients can control cache behavior for their own connection by setting the SESSION value of query_cache_type. For example, a client can disable use of the query cache for its own queries like this:

mysql> SET SESSION query_cache_type = OFF;

To control the maximum size of individual query results that can be cached, set the query_cache_limit variable. The default value is 1MB.

The result of a query (the data sent to the client) is stored in the query cache during result retrieval. Therefore the data usually is not handled in one big chunk. The query cache allocates blocks for storing this data on demand, so when one block is filled, a new block is allocated. Because memory allocation operation is costly (timewise), the query cache allocates blocks with a minimum size given by the query_cache_min_res_unit system variable. When a query is executed, the last result block is trimmed to the actual data size so that unused memory is freed. Depending on the types of queries your server executes, you might find it helpful to tune the value of query_cache_min_res_unit:

query_cache_min_res_unit is present from MySQL 4.1.

5.11.4 Query Cache Status and Maintenance

You can check whether the query cache is present in your MySQL server using the following statement:

mysql> SHOW VARIABLES LIKE 'have_query_cache';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| have_query_cache | YES   |
+------------------+-------+

You can defragment the query cache to better utilize its memory with the FLUSH QUERY CACHE statement. The statement does not remove any queries from the cache.

The RESET QUERY CACHE statement removes all query results from the query cache. The FLUSH TABLES statement also does this.

To monitor query cache performance, use SHOW STATUS to view the cache status variables:

mysql> SHOW STATUS LIKE 'Qcache%';
+-------------------------+--------+
| Variable_name           | Value  |
+-------------------------+--------+
| Qcache_free_blocks      | 36     |
| Qcache_free_memory      | 138488 |
| Qcache_hits             | 79570  |
| Qcache_inserts          | 27087  |
| Qcache_lowmem_prunes    | 3114   |
| Qcache_not_cached       | 22989  |
| Qcache_queries_in_cache | 415    |
| Qcache_total_blocks     | 912    |
+-------------------------+--------+

Descriptions of each of these variables are given in section 5.2.4 Server Status Variables. Some uses for them are described here.

The total number of SELECT queries is equal to:

  Com_select
+ Qcache_hits
+ queries with errors found by parser

The Com_select value is equal to:

  Qcache_inserts
+ Qcache_not_cached
+ queries with errors found during columns/rights check

The query cache uses variable-length blocks, so Qcache_total_blocks and Qcache_free_blocks may indicate query cache memory fragmentation. After FLUSH QUERY CACHE, only a single free block remains.

Every cached query requires a minimum of two blocks (one for the query text and one or more for the query results). Also, every table that is used by a query requires one block. However, if two or more queries use the same table, only one block needs to be allocated.

The information provided by the Qcache_lowmem_prunes status variable can help you tune the query cache size. It counts the number of queries that have been removed from the cache to free up memory for caching new queries. The query cache uses a least recently used (LRU) strategy to decide which queries to remove from the cache. Tuning information is given in section 5.11.3 Query Cache Configuration.


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