MySQL Database Administration

If you need to administer MySQL, this article gets you off to a good start. The first of a multi-part series, it is excerpted from chapter four of the book MySQL Administrator’s Guide, written by Paul Dubois (Sams; ISBN: 0672326345).

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

4.1 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 4.2, “Configuring the MySQL Server.”

4.1.1 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. See Section 3.3, “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 4.2, “Configuring the MySQL Server.”

  • mysqld-max

    A version of the server that includes additional features. See Section 4.1.2, “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 4.1.3, “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 4.1.4, “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 4.1.5, “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 4.6.2, “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.)

{mospagebreak title=4.1.2 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:

  • For Windows, MySQL binary distributions include both the standard server (mysqld.exe) and the MySQL-Max server (mysqld-max.exe), so you need not get a special distribution. Just use a regular Windows distribution, available at http://dev.mysql.com/downloads/mysql-4.0.html. See Section 2.2.1, “Installing MySQL on Windows.”

  • For Linux, if you install MySQL using RPM distributions, use the regular MySQL-server RPM first to install a standard server named mysqld. Then use the MySQL-Max RPM to install a server named mysqld-max. The MySQL-Max RPM presupposes that you have already installed the regular server RPM. See Section 2.2.2, “Installing MySQL on Linux,” for more information on the Linux RPM packages.

  • All other MySQL-Max distributions contain a single server that is named mysqld but that has the additional features included.

You can find the MySQL-Max binaries on the MySQL AB Web site at http://dev.mysql.com/downloads/mysql-max-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 9.3, “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 8.4.3, “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.

{mospagebreak title=4.1.3 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:

  • On Linux, the MySQL-Max RPM relies on this mysqld_safe behavior. The RPM installs an executable named mysqld-max, which causes mysqld_safe to automatically use that executable from that point on.

  • If you install a MySQL-Max distribution that includes a server named mysqld-max, then upgrade later to a non-Max version of MySQL, mysqld_safe will still attempt to run the old mysqld-max server. If you perform such an upgrade, you should manually remove the old mysqld-max server to ensure that mysqld_safe runs the new mysqld server.

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 4.2.1, “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 3.3.2, “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 4.8.1, “The Error Log.”

  • --mysqld=prog_name

    The name of the server program (in the ledir directory) that you want to start.

  • --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:

  • The server and databases can be found relative to the directory from which mysqld_safe is invoked. For binary distributions, mysqld_safe looks under its working directory for bin and data directories. For source distributions, it looks for libexec and var directories. This condition should be met if you execute mysqld_safe from your MySQL installation directory (for example, /usr/local/mysql for a binary distribution).

  • If the server and databases cannot be found relative to the working directory, mysqld_safe attempts to locate them by absolute pathnames. Typical locations are /usr/local/libexec and
    /usr/local/var. The actual locations are determined from the values configured into the distribution at the time it was built. They should be correct if MySQL is installed in the location specified at configuration time.

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.

{mospagebreak title=4.1.4 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 2.2.2, “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 use a binary distribution format that does not install mysql.server automatically, you can install it manually. Instructions are provided in Section 2.4.3, “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.

4.1.5 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 2.4.3, “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 4.9, “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 start 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.

  • --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.

  • --version

    Display version information and exit.

Some notes about mysqld_multi:

  • Make sure that the MySQL account used for stopping the mysqld servers (with the mysqladmin program) has the same username and password for each server. Also, make sure that the account has the SHUTDOWN privilege. If the servers that you want to manage have many different usernames or passwords for the administrative accounts, you might want to create an account on each server that has the same username and password. For example, you might set up a common multi_admin account by executing the following commands for each server:

    shell> mysql -u root -S /tmp/mysql.sock
    -p
    root_password mysql> GRANT SHUTDOWN ON *.*
    -> TO 'multi_admin'@'localhost' IDENTIFIED
    BY 'multipass';

    See Section 4.4.2, “How the Privilege System Works.” You will have to do this for each mysqld server. Change the connection parameters appropriately when connecting to each one. Note that the host part of the account name must allow you to connect as multi_admin from the host where you want to run mysqld_multi.

  • The --pid-file option is very important if you are using mysqld_safe to start mysqld (for example, --mysqld=mysqld_safe) Every mysqld should have its own process ID file. The advantage of using mysqld_safe instead of mysqld is that mysqld_safe “guards” its mysqld process and will restart it if the process terminates due to a signal sent using kill -9, or for other reasons, such as a segmentation fault. Please note that the mysqld_safe script might require that you start it from a certain place. This means that you might have to change location to a certain directory before running mysqld_multi. If you have problems starting, please see the mysqld_safe script. Check especially the lines:

    -----------------------------------------------------
    MY_PWD=´pwd´
    # Check if we are starting this relative (for
    the binary release) if test -d $MY_PWD/data/mysql -a -f ./share/
    mysql/english/errmsg.sys -a -x ./bin/mysqld -----------------------------------------------------

    See Section 4.1.3, “The mysqld_safe Server Startup Script.” The test performed by these lines should be successful, or you might encounter problems.

  • The Unix socket file and the TCP/IP port number must be different for every mysqld.

  • You might want to use the --user option for mysqld, but in order to do this you need to run the mysqld_multi script as the Unix root user. Having the option in the option file doesn’t matter; you will just get a warning, if you are not the superuser and the mysqld processes are started under your own Unix account.

  • Important: Make sure that the data directory is fully accessible to the Unix account that the specific mysqld process is started as. Do not use the Unix root account for this, unless you know what you are doing.

  • Most important: Before using mysqld_multi be sure that you understand the meanings of the options that are passed to the mysqld servers and why you would want to have separate mysqld processes. Beware of the dangers of using multiple mysqld servers with the same data directory. Use separate data directories, unless you know what you are doing. Starting multiple servers with the same data directory will not give you extra performance in a threaded system. See Section 4.9, “Running Multiple MySQL Servers on the Same Machine.”

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 3.3.2, “Using Option Files.”

{mospagebreak title=4.2 Configuring the MySQL Server}

This section discusses MySQL server configuration topics:

  • Startup options that the server supports

  • How to set the server SQL mode

  • Server system variables

  • Server status variables

4.2.1 mysqld Command-Line Options

When you start the mysqld server, you can specify program options using any of the methods described in Section 3.3, “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 3.3.2, “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:

  • Options that affect security: See Section 4.3.3, “Startup Options for mysqld Concerning Security.”

  • SSL-related options: See Section 4.5.7.5, “SSL Command-Line Options.”

  • Binary log control options: See Section 4.8.4, “The Binary Log.”

  • Replication-related options: See Section 5.8, “Replication Startup Options.”

  • Options specific to particular storage engines: See Section 8.1.1, “MyISAM Startup Options,” Section 8.4.3, “BDB Startup Options,” and Section 9.5, “InnoDB Startup Options.”

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 1.8.3, “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 4.7.1, “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.

  • --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 4.1.3, “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.

  • --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'.

  • --default-character-set=charset

    Use charset as the default character set. See Section 4.7.1, “The Character Set Used for Data and Sorting.”

  • --default-collation=collation

    Use collation as the default collation. This option is available as of MySQL 4.1.1. See Section 4.7.1, “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 Chapter 8, “MySQL Storage Engines and Table Types.”

  • --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 6.5.2, “Tuning Server Parameters.” See Section 8.1.1, “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, and XP systems, and can be used only with the mysqld-nt and mysqld-max-nt servers that support named pipe connections.

  • --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:

    • You should not use the query cache for queries that use tables that are updated by another process.

    • You should not use --delay-key-write=ALL or DELAY_KEY_WRITE=1 on any shared tables.

    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.)

  • --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!

  • --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.

  • --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 4.7.2, “Setting the Error Message Language.”

  • --log[=file], -l [file]

    Log connections and queries to this file. See Section 4.8.2, “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 4.8.4, “The Binary Log.” If you don’t specify a filename, MySQL will use host_name-bin as the filename.

  • --log-bin-index[=file]

    The index file for binary log filenames. See Section 4.8.4, “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 4.8.1, “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, then 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 4.8.5, “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 file. See Section 4.8.5, “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 4.8.3, “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 4.8.4, “The Binary Log.” Starting from version 5.0.0, using --log-update will just turn on the binary log instead.

  • --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 4.1.2; to disable it, use --skip-log-warnings. 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 6.3.2, “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 normally is 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, 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 you 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.

  • --new

    From version 4.0.12, 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:

    • TIMESTAMP is returned as a string with the format 'YYYY-MM-DD HH:MM:SS'.

      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 2.5.6, “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.

  • --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 4.4.3, “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.

  • --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 6.5.2, “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 6.5.5, “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 6.5.5, “How MySQL Uses DNS.”

  • --skip-networking

    Don’t listen for TCP/IP connections at all. All interaction with mysqld must be made via named pipes (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are allowed. See Section 6.5.5, “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:

    • On Windows, enabling symbolic links allows you to establish a symbolic link to a database directory by creating a directory.sym file that contains the path to the real directory. See Section 6.6.1.3, “Using Symbolic Links for Databases on Windows.”

    • On Unix, enabling symbolic links means that you can link a MyISAM index file or data file to another directory with the INDEX DIRECTORY or DATA DIRECTORY options of the CREATE TABLE statement. If you delete or rename the table, the files that its symbolic links point to also are deleted or renamed.

    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.

  • --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 4.2.2, “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.

  • --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 4.3.1, “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.

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 4.2.3, “Server System Variables.” The section on tuning server parameters includes information on how to optimize them. See Section 6.5.2, “Tuning Server Parameters.”

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

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.

{mospagebreak title=4.2.2 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 applications to tailor server operation to their 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. Beginning with MySQL 4.1, you can also change the mode after startup time by setting the sql_mode variable with a SET [SESSION|GLOBAL] sql_mode='modes' statement. Setting the GLOBAL variable affects the operation of all clients that connect from that time on. Setting the SESSION variable affects only the current client. 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 value also can be empty (--sql-mode="") if you want to reset it.

The following list describes the supported modes:

  • 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.)

  • 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_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. 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, normally MySQL 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 statements in the dump output to enable NO_AUTO_VALUE_ON_ZERO. (New in MySQL 4.1.1.)

  • 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. (New in MySQL 4.0.2.)

  • 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.)

The following special modes are provided as shorthand for combinations of mode values from the preceding list. They are available as of MySQL 4.1.1.

  • ANSI

    Equivalent to REAL_AS_FLOAT, PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, ONLY_FULL_GROUP_BY. See Section 1.8.3, “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.

  • 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.

  • MYSQL40

    Equivalent to NO_FIELD_OPTIONS.

  • ORACLE

    Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.

  • POSTGRESQL

    Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.

{mospagebreak title=4.2.3 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 4.2.3.1.2, “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 4.2.3.1, “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                    | OFF                          |
| 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 in Section 9.5, “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 6.5.2, “Tuning Server Parameters.”

  • ansi_mode

    This is ON if mysqld was started with --ansi. See Section 1.8.3, “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

    The BDB storage engine version. This variable was added in MySQL 3.23.31.

  • 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 4.8.4, “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, for some functions, 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.

  • 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_inserts

    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.

  • 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 4.2.1, “mysqld Command-Line Options,” and Section 8.1.1, “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

    How many 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.

  • 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.

    The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:

    • Operator function is determined by position within the string.

    • The replacement value must be 14 characters.

    • Each character must be an ASCII non-alphanumeric character.

    • Either the first or second character must be a space.

    • No duplicates are allowed except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.

    • Positions 10, 13, and 14 (which by default are set to ‘:‘, ‘&‘, and ‘|‘) are reserved for future extensions.

  • 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_bdb

    YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used. This variable was added in MySQL 3.23.30.

  • 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_raid

    YES if mysqld supports the RAID option. This variable was added in MySQL 3.23.30.

  • have_openssl

    YES if mysqld supports SSL (encryption) of the client/server protocol. This variable was added in MySQL 3.23.43.

  • 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'

    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 in Section 9.5, “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). The buffer is allocated one time for each full join between two tables. Increase this value to get a faster full join when adding indexes is not possible. (Normally the best way to get fast joins is to add indexes.)

  • 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.

    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.

    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.

    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_used 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:

    (Key_blocks_used * key_cache_block_size) /
    key_buffer_size
  • Before MySQL 4.1.1, key cache blocks are 1024 bytes, so the fraction of the key buffer in use is:

    (Key_blocks_used * 1024) / key_buffer_size

    See Section 6.4.6, “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 6.4.6, “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 6.4.6, “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 6.4.6, “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.

  • 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 4.8.2, “The General Query Log.”

  • log_bin

    Whether the binary log is enabled. This variable was added in MySQL 3.23.14. See Section 4.8.4, “The Binary Log.”

  • 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 5.8, “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 4.8.5, “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 4.8.3, “The Update Log.”

  • 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 4.8.5, “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_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.

    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 0 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. The 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. 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 6.4.8, “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_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 5.3, “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. 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

    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.

  • 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.

  • 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.

  • 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 4.10.3, “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.

  • 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.

  • 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. 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_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. See Section 4.2.2, “The Server SQL Mode.”

  • storage_engine

    This variable is a synonym for table_type. It was added in MySQL 4.1.2.

  • 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 4.2.4, “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 6.4.8, “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 4.2.1, “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 4.2.4, “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 6.1.4, “The MySQL Benchmark Suite.”

  • 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. 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.

  • version

    The version number for the server.

  • 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.

Please check back next week for the continuation of this article.

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