To see the values that a server uses based on its compiled-in defaults and any option files that it reads, use this command:

mysqld --verbose --help

To see the values that a server uses based only on its compiled-in defaults, ignoring the settings in any option files, use this command:

mysqld --no-defaults --verbose --help

To see the current values used by a running server, use the SHOW VARIABLES statement or the Performance Schema system variable tables. See Section 29.12.14, “Performance Schema System Variable Tables”.

Section 7.1.9, “Using System Variables”, discusses the syntax for setting and displaying system variable values.

Section 7.1.9.2, “Dynamic System Variables”, lists the variables that can be set at runtime.

Information on tuning system variables can be found in Section 7.1.1, “Configuring the Server”.

Section 17.14, “InnoDB Startup Options and System Variables”, lists InnoDB system variables.

Section 25.4.3.9.2, “NDB Cluster System Variables”, lists system variables which are specific to NDB Cluster.

For information on server system variables specific to replication, see Section 19.1.6, “Replication and Binary Logging Options and Variables”.

activate_all_roles_on_login

Whether to enable automatic activation of all granted roles when users log in to the server:

Granted roles include those granted explicitly to the user and those named in the mandatory_roles system variable value.

activate_all_roles_on_login applies only at login time, and at the beginning of execution for stored programs and views that execute in definer context. To change the active roles within a session, use SET ROLE. To change the active roles for a stored program, the program body should execute SET ROLE.

admin_address

The IP address on which to listen for TCP/IP connections on the administrative network interface (see Section 7.1.12.1, “Connection Interfaces”). There is no default admin_address value. If this variable is not specified at startup, the server maintains no administrative interface. The server also has a bind_address system variable for configuring regular (nonadministrative) client TCP/IP connections. See Section 7.1.12.1, “Connection Interfaces”.

If admin_address is specified, its value must satisfy these requirements:

An IP address can be specified as an IPv4 or IPv6 address. If the value is a host name, the server resolves the name to an IP address and binds to that address. If a host name resolves to multiple IP addresses, the server uses the first IPv4 address if there are any, or the first IPv6 address otherwise.

The server treats different types of addresses as follows:

These rules apply to specifying a network namespace for an address:

For additional information about network namespaces, see Section 7.1.14, “Network Namespace Support”.

If binding to the address fails, the server produces an error and does not start.

The admin_address system variable is similar to the bind_address system variable that binds the server to an address for ordinary client connections, but with these differences:

admin_port

The TCP/IP port number to use for connections on the administrative network interface (see Section 7.1.12.1, “Connection Interfaces”). Setting this variable to 0 causes the default value to be used.

Setting admin_port has no effect if admin_address is not specified because in that case the server maintains no administrative network interface.

admin_ssl_ca

The admin_ssl_ca system variable is like ssl_ca, except that it applies to the administrative connection interface rather than the main connection interface. For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.

admin_ssl_capath

The admin_ssl_capath system variable is like ssl_capath, except that it applies to the administrative connection interface rather than the main connection interface. For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.

admin_ssl_cert

The admin_ssl_cert system variable is like ssl_cert, except that it applies to the administrative connection interface rather than the main connection interface. For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.

admin_ssl_cipher

The admin_ssl_cipher system variable is like ssl_cipher, except that it applies to the administrative connection interface rather than the main connection interface. For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.

The list specified by this variable may include any of the following values:

Trying to include any values in the cipher list that are not shown here when setting this variable raises an error (ER_BLOCKED_CIPHER).

admin_ssl_crl

The admin_ssl_crl system variable is like ssl_crl, except that it applies to the administrative connection interface rather than the main connection interface. For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.

admin_ssl_crlpath

The admin_ssl_crlpath system variable is like ssl_crlpath, except that it applies to the administrative connection interface rather than the main connection interface. For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.

admin_ssl_key

The admin_ssl_key system variable is like ssl_key, except that it applies to the administrative connection interface rather than the main connection interface. For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.

admin_tls_ciphersuites

The admin_tls_ciphersuites system variable is like tls_ciphersuites, except that it applies to the administrative connection interface rather than the main connection interface. For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.

The value is a list of zero or more colon-separated ciphersuite names from among those listed here:

Trying to include any values in the cipher list that are not shown here when setting this variable raises an error (ER_BLOCKED_CIPHER).

admin_tls_version

The admin_tls_version system variable is like tls_version, except that it applies to the administrative connection interface rather than the main connection interface. For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.

authentication_openid_connect_configuration

Specifies a list of issuers and their corresponding public signing keys used to validate the Identity token's signature for Section 8.4.1.9, “OpenID Connect Pluggable Authentication”. It accepts a JSON string with the JSON:// prefix, or a full path to a JSON file with the file:// prefix.

authentication_policy

This variable is used to administer multifactor authentication (MFA) capabilities. For CREATE USER and ALTER USER statements used to manage MySQL account definitions, it determines what authentication factor or factors may be specified, where “factor” corresponds to an authentication method or plugin associated with an account. authentication_policy determines the following aspects of multifactor authentication:

Because authentication_policy applies only when accounts are created or altered, changes to its value have no effect on existing user accounts.

The value of authentication_policy is a list of 1, 2, or 3 comma-separated elements, each corresponding to an authentication factor and each being of one of the forms listed here, with their meanings:

In each case, an element may be surrounded by whitespace characters. The entire list must be enclosed in single quotes.

authentication_policy must contain at least one nonempty factor, and any empty factors must come at the end of the list, following any nonempty factors. This means that ',,' is not permitted because this signifies that all factors are optional. Every account must have at least one authentication factor.

The default value of authentication_policy is '*,,'. This means that factor 1 is required in account definitions and can use any authentication plugin (with caching_sha2_password being the default), and that factors 2 and 3 are optional and each can use any authentication plugin.

If authentication_policy does not specify a default plugin for the first factor, the default plugin for this factor is caching_sha2_password, although another plugin may be used.

The following table shows some possible values for authentication_policy and the policy that each establishes for creating or altering accounts.

authentication_windows_log_level

This variable is available only if the authentication_windows Windows authentication plugin is enabled and debugging code is enabled. See Section 8.4.1.5, “Windows Pluggable Authentication”.

This variable sets the logging level for the Windows authentication plugin. The following table shows the permitted values.

authentication_windows_use_principal_name

This variable is available only if the authentication_windows Windows authentication plugin is enabled. See Section 8.4.1.5, “Windows Pluggable Authentication”.

A client that authenticates using the InitSecurityContext() function should provide a string identifying the service to which it connects (targetName). MySQL uses the principal name (UPN) of the account under which the server is running. The UPN has the form user_id@computer_name and need not be registered anywhere to be used. This UPN is sent by the server at the beginning of authentication handshake.

This variable controls whether the server sends the UPN in the initial challenge. By default, the variable is enabled. For security reasons, it can be disabled to avoid sending the server's account name to a client as cleartext. If the variable is disabled, the server always sends a 0x00 byte in the first challenge, the client does not specify targetName, and as a result, NTLM authentication is used.

If the server fails to obtain its UPN (which happens primarily in environments that do not support Kerberos authentication), the UPN is not sent by the server and NTLM authentication is used.

autocommit

The autocommit mode. If set to 1, all changes to a table take effect immediately. If set to 0, you must use COMMIT to accept a transaction or ROLLBACK to cancel it. If autocommit is 0 and you change it to 1, MySQL performs an automatic COMMIT of any open transaction. Another way to begin a transaction is to use a START TRANSACTION or BEGIN statement. See Section 15.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Statements”.

By default, client connections begin with autocommit set to 1. To cause clients to begin with a default of 0, set the global autocommit value by starting the server with the --autocommit=0 option. To set the variable using an option file, include these lines:

[mysqld]
autocommit=0

automatic_sp_privileges

When this variable has a value of 1 (the default), the server automatically grants the EXECUTE and ALTER ROUTINE privileges to the creator of a stored routine, if the user cannot already execute and alter or drop the routine. (The ALTER ROUTINE privilege is required to drop the routine.) The server also automatically drops those privileges from the creator when the routine is dropped. If automatic_sp_privileges is 0, the server does not automatically add or drop these privileges.

The creator of a routine is the account used to execute the CREATE statement for it. This might not be the same as the account named as the DEFINER in the routine definition.

If you start mysqld with --skip-new, automatic_sp_privileges is set to OFF.

The effects of this variable are the same with regard to JavaScript libraries as they are for stored routines; when automatic_sp_privileges is 1, the EXECUTE and ALTER ROUTINE privileges are given automatically to the creator of a library if the creator does not already have those privileges.

See also Section 27.2.2, “Stored Routines and MySQL Privileges”.

auto_generate_certs

This variable controls whether the server autogenerates SSL key and certificate files in the data directory, if they do not already exist.

At startup, the server automatically generates server-side and client-side SSL certificate and key files in the data directory if the auto_generate_certs system variable is enabled and the server-side SSL files are missing from the data directory. These certificates are always generated in such cases, regardless of the values of any other TLS options. The certificate and key files enable secure client connections using SSL; see Section 8.3.1, “Configuring MySQL to Use Encrypted Connections”.

For more information about SSL file autogeneration, including file names and characteristics, see Section 8.3.3.1, “Creating SSL and RSA Certificates and Keys using MySQL”

The sha256_password_auto_generate_rsa_keys and caching_sha2_password_auto_generate_rsa_keys system variables are related but control autogeneration of RSA key-pair files needed for secure password exchange using RSA over unencrypted connections.

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. back_log cannot be set higher than your operating system limit.

The default value is the value of max_connections, which enables the permitted backlog to adjust to the maximum permitted number of connections.

basedir

The path to the MySQL installation base directory.

big_tables

If enabled, the server stores all temporary tables on disk rather than in memory. This prevents most The table tbl_name is full errors for SELECT operations that require a large temporary table, but also slows down queries for which in-memory tables would suffice.

The default value for new connections is OFF (use in-memory temporary tables). Normally, it should never be necessary to enable this variable. When in-memory internal temporary tables are managed by the TempTable storage engine (the default), and the maximum amount of memory that can be occupied by the TempTable storage engine is exceeded, the TempTable storage engine starts storing data to temporary files on disk. When in-memory temporary tables are managed by the MEMORY storage engine, in-memory tables are automatically converted to disk-based tables as required. For more information, see Section 10.4.4, “Internal Temporary Table Use in MySQL”.

bind_address

The MySQL server listens on one or more network sockets for TCP/IP connections. Each socket is bound to one address, but it is possible for an address to map onto multiple network interfaces. To specify how the server should listen for TCP/IP connections, set the bind_address system variable at server startup. The server also has an admin_address system variable that enables administrative connections on a dedicated interface. See Section 7.1.12.1, “Connection Interfaces”.

If bind_address is specified, it accepts a list of one or more address values, each of which may specify a single non-wildcard IP address or host name. Each address may include a network namespace specifier. If only one address is specified, it may make use of one of the wildcard address formats that permit listening on multiple network interfaces (*, 0.0.0.0, or ::). Multiple addresses are separated by commas. When multiple values are listed, each value must specify a single non-wildcard IP address (either IPv4 or IPv6) or a host name, and wildcard address formats (*, 0.0.0.0, or ::) are not allowed.

IP addresses can be specified as IPv4 or IPv6 addresses. For any value that is a host name, the server resolves the name to an IP address and binds to that address. If a host name resolves to multiple IP addresses, the server uses the first IPv4 address if there are any, or the first IPv6 address otherwise.

The server treats different types of addresses as follows:

These rules apply to specifying a network namespace for an address:

For additional information about network namespaces, see Section 7.1.14, “Network Namespace Support”.

If binding to any address fails, the server produces an error and does not start.

Examples:

When bind_address names a single value (wildcard or non-wildcard), the server listens on a single socket, which for a wildcard address may be bound to multiple network interfaces. When bind_address names a list of multiple values, the server listens on one socket per value, with each socket bound to a single network interface. The number of sockets is linear with the number of values specified. Depending on operating system connection-acceptance efficiency, long value lists might incur a performance penalty for accepting TCP/IP connections.

Because file descriptors are allocated for listening sockets and network namespace files, it may be necessary to increase the open_files_limit system variable.

If you intend to bind the server to a specific address, be sure that the mysql.user system table contains an account with administrative privileges that you can use to connect to that address. Otherwise, you cannot shut down the server. For example, if you bind the server to *, you can connect to it using all existing accounts. But if you bind the server to ::1, it accepts connections only on that address. In that case, first make sure that the 'root'@'::1' account is present in the mysql.user table so you can still connect to the server to shut it down.

block_encryption_mode

This variable controls the block encryption mode for block-based algorithms such as AES. It affects encryption for AES_ENCRYPT() and AES_DECRYPT().

block_encryption_mode takes a value in aes-keylen-mode format, where keylen is the key length in bits and mode is the encryption mode. The value is not case-sensitive. Permitted keylen values are 128, 192, and 256. Permitted mode values are ECB, CBC, CFB1, CFB8, CFB128, and OFB.

For example, this statement causes the AES encryption functions to use a key length of 256 bits and the CBC mode:

SET block_encryption_mode = 'aes-256-cbc';

An error occurs for attempts to set block_encryption_mode to a value containing an unsupported key length or a mode that the SSL library does not support.

build_id

This is a 160-bit SHA1 signature which is generated by the linker when compiling the server on Linux systems with -DWITH_BUILD_ID=ON (enabled by default), and converted to a hexadecimal string. This read-only value serves as a unique build ID, and is written into the server log at startup.

build_id is not supported on platforms other than Linux.

bulk_insert_buffer_size

MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA when adding data to nonempty tables. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. The default value is 8MB.

Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 7.1.9.1, “System Variable Privileges”.

caching_sha2_password_digest_rounds

The number of hash rounds used by the caching_sha2_password authentication plugin for password storage.

Increasing the number of hashing rounds above the default value incurs a performance penalty that correlates with the amount of increase:

caching_sha2_password_auto_generate_rsa_keys

The server uses this variable to determine whether to autogenerate RSA private/public key-pair files in the data directory if they do not already exist.

At startup, the server automatically generates RSA private/public key-pair files in the data directory if all of these conditions are true: The sha256_password_auto_generate_rsa_keys or caching_sha2_password_auto_generate_rsa_keys system variable is enabled; no RSA options are specified; the RSA files are missing from the data directory. These key-pair files enable secure password exchange using RSA over unencrypted connections for accounts authenticated by the sha256_password (deprecated) or caching_sha2_password plugin; see Section 8.4.1.2, “SHA-256 Pluggable Authentication”, and Section 8.4.1.1, “Caching SHA-2 Pluggable Authentication”.

For more information about RSA file autogeneration, including file names and characteristics, see Section 8.3.3.1, “Creating SSL and RSA Certificates and Keys using MySQL”

The auto_generate_certs system variable is related but controls autogeneration of SSL certificate and key files needed for secure connections using SSL.

caching_sha2_password_private_key_path

This variable specifies the path name of the RSA private key file for the caching_sha2_password authentication plugin. If the file is named as a relative path, it is interpreted relative to the server data directory. The file must be in PEM format.

For information about caching_sha2_password, see Section 8.4.1.1, “Caching SHA-2 Pluggable Authentication”.

caching_sha2_password_public_key_path

This variable specifies the path name of the RSA public key file for the caching_sha2_password authentication plugin. If the file is named as a relative path, it is interpreted relative to the server data directory. The file must be in PEM format.

For information about caching_sha2_password, including information about how clients request the RSA public key, see Section 8.4.1.1, “Caching SHA-2 Pluggable Authentication”.

character_set_client

The character set for statements that arrive from the client. The session value of this variable is set using the character set requested by the client when the client connects to the server. (Many clients support a --default-character-set option to enable this character set to be specified explicitly. See also Section 12.4, “Connection Character Sets and Collations”.) The global value of the variable is used to set the session value in cases when the client-requested value is unknown or not available, or the server is configured to ignore client requests. This can happen when the client requests a character set not known to the server, such as when a Japanese-enabled client requests sjis when connecting to a server not configured with sjis support.

Some character sets cannot be used as the client character set. Attempting to use them as the character_set_client value produces an error. See Impermissible Client Character Sets.

character_set_connection

The character set used for literals specified without a character set introducer and for number-to-string conversion. For information about introducers, see Section 12.3.8, “Character Set Introducers”.

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.

Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 7.1.9.1, “System Variable Privileges”.

The global character_set_database and collation_database system variables are deprecated; expect them to be removed in a future version of MySQL.

Assigning a value to the session character_set_database and collation_database system variables is deprecated and assignments produce a warning. Expect the session variables to become read-only (and assignments to them to produce an error) in a future version of MySQL in which it remains possible to access the session variables to determine the database character set and collation for the default database.

character_set_filesystem

The file system character set. This variable is used to interpret string literals that refer to file names, such as in the LOAD DATA and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function. Such file names are converted from character_set_client to character_set_filesystem before the file opening attempt occurs. The default value is binary, which means that no conversion occurs. For systems on which multibyte file names are permitted, a different value may be more appropriate. For example, if the system represents file names using UTF-8, set character_set_filesystem to 'utf8mb4'.

Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 7.1.9.1, “System Variable Privileges”.

character_set_results

The character set used for returning query results to the client. This includes result data such as column values, result metadata such as column names, and error messages.

character_set_server

The servers default character set. See Section 12.15, “Character Set Configuration”. If you set this variable, you should also set collation_server to specify the collation for the character set.

character_set_system

The character set used by the server for storing identifiers. The value is always utf8mb3.

character_sets_dir

The directory where character sets are installed. See Section 12.15, “Character Set Configuration”.

check_proxy_users

Some authentication plugins implement proxy user mapping for themselves (for example, the PAM and Windows authentication plugins). Other authentication plugins do not support proxy users by default. Of these, some can request that the MySQL server itself map proxy users according to granted proxy privileges: mysql_native_password (deprecated), sha256_password (deprecated).

If the check_proxy_users system variable is enabled, the server performs proxy user mapping for any authentication plugins that make such a request. To take advantage of server proxy user mapping support for the sha256_password plugin, you must enable sha256_password_proxy_users.

For information about user proxying, see Section 8.2.19, “Proxy Users”.

collation_connection

The collation of the connection character set. collation_connection is important for comparisons of literal strings. For comparisons of strings with column values, collation_connection does not matter because columns have their own collation, which has a higher collation precedence (see Section 12.8.4, “Collation Coercibility in Expressions”).

Using the name of a user-defined collation for this variable raises a warning.

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.

The global character_set_database and collation_database system variables are deprecated; expect them to be removed in a future version of MySQL.

Assigning a value to the session character_set_database and collation_database system variables is deprecated and assignments produce a warning. Expect the session variables to become read-only (and assignments to produce an error) in a future version of MySQL in which it remains possible to access the session variables to determine the database character set and collation for the default database.

Using the name of a user-defined collation for collation_database raises a warning.

collation_server

The server's default collation. See Section 12.15, “Character Set Configuration”.

Setting this to the name of a user-defined collation raises a warning.

completion_type

The transaction completion type. This variable can take the values shown in the following table. The variable can be assigned using either the name values or corresponding integer values.

completion_type affects transactions that begin with START TRANSACTION or BEGIN and end with COMMIT or ROLLBACK. It does not apply to implicit commits resulting from execution of the statements listed in Section 15.3.3, “Statements That Cause an Implicit Commit”. It also does not apply for XA COMMIT, XA ROLLBACK, or when autocommit=1.

component_scheduler.enabled

When set to OFF at startup, the background thread does not start. Tasks can still be scheduled, but they do not run until component_scheduler is enabled. When set to ON at startup, the component is fully operational.

It is also possible to set the value dynamically to get the following effects:

concurrent_insert

If AUTO (the default), MySQL permits INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle of the data file.

This variable can take the values shown in the following table. The variable can be assigned using either the name values or corresponding integer values.

If you start mysqld with --skip-new, concurrent_insert is set to NEVER.

See also Section 10.11.3, “Concurrent Inserts”.

connect_timeout

The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake. The default value is 10 seconds.

Increasing the connect_timeout value might help if clients frequently encounter errors of the form Lost connection to MySQL server at 'XXX', system error: errno.

connection_memory_chunk_size

Set the chunking size for updates to the global memory usage counter Global_connection_memory. The status variable is updated only when total memory consumption by all user connections changes by more than this amount. Disable updates by setting connection_memory_chunk_size = 0.

The memory calculation is exclusive of any memory used by system users such as the MySQL root user. Memory used by the InnoDB buffer pool is also not included.

You must have the SYSTEM_VARIABLES_ADMIN or SUPER privilege to set this variable.

connection_memory_limit

Set the maximum amount of memory that can be used by a single user connection. If any user connection uses more than this amount, all queries from this connection are rejected with ER_CONN_LIMIT, including any queries currently running.

The limit set by this variable does not apply to system users, or to the MySQL root account. Memory used by the InnoDB buffer pool is also not included.

You must have the SYSTEM_VARIABLES_ADMIN or SUPER privilege to set this variable.

connection_memory_status_limit

This is the maximum amount of memory that can be consumed by any one user connection before before Count_hit_query_past_connection_memory_status_limit is incremented.

core_file

Whether to write a core file if the server unexpectedly exits. This variable is set by the --core-file option.

create_admin_listener_thread

Whether to use a dedicated listening thread for client connections on the administrative network interface (see Section 7.1.12.1, “Connection Interfaces”). The default is OFF; that is, the manager thread for ordinary connections on the main interface also handles connections for the administrative interface.

Depending on factors such as platform type and workload, you may find one setting for this variable yields better performance than the other setting.

Setting create_admin_listener_thread has no effect if admin_address is not specified because in that case the server maintains no administrative network interface.

cte_max_recursion_depth

The common table expression (CTE) maximum recursion depth. The server terminates execution of any CTE that recurses more levels than the value of this variable. For more information, see Limiting Common Table Expression Recursion.

datadir

The path to the MySQL server data directory. Relative paths are resolved with respect to the current directory. If you expect the server to be started automatically (that is, in contexts for which you cannot know the current directory in advance), it is best to specify the datadir value as an absolute path.

debug

This variable indicates the current debugging settings. It is available only for servers built with debugging support. The initial value comes from the value of instances of the --debug option given at server startup. The global and session values may be set at runtime.

Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 7.1.9.1, “System Variable Privileges”.

Assigning a value that begins with + or - cause the value to added to or subtracted from the current value:

mysql> SET debug = 'T';
mysql> SELECT @@debug;
+---------+
| @@debug |
+---------+
| T       |
+---------+

mysql> SET debug = '+P';
mysql> SELECT @@debug;
+---------+
| @@debug |
+---------+
| P:T     |
+---------+

mysql> SET debug = '-P';
mysql> SELECT @@debug;
+---------+
| @@debug |
+---------+
| T       |
+---------+

For more information, see Section 7.9.4, “The DBUG Package”.

debug_sync

This variable is the user interface to the Debug Sync facility. Use of Debug Sync requires that MySQL be configured with the -DWITH_DEBUG=ON CMake option (see Section 2.8.7, “MySQL Source-Configuration Options”); otherwise, this system variable is not available.

The global variable value is read only and indicates whether the facility is enabled. By default, Debug Sync is disabled and the value of debug_sync is OFF. If the server is started with --debug-sync-timeout=N, where N is a timeout value greater than 0, Debug Sync is enabled and the value of debug_sync is ON - current signal followed by the signal name. Also, N becomes the default timeout for individual synchronization points.

The session value can be read by any user and has the same value as the global variable. The session value can be set to control synchronization points.

Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 7.1.9.1, “System Variable Privileges”.

For a description of the Debug Sync facility and how to use synchronization points, see MySQL Server Doxygen Documentation.

default_collation_for_utf8mb4

This variable is set by the server to the default collation for the utf8mb4 character set. The value of the variable is replicated from a source to a replica so that the replica can correctly process data originating from a source with a different default collation for utf8mb4. This variable is primarily intended to support replication from a MySQL 5.7 or older replication source server to a later MySQL replica server, or group replication with a MySQL 5.7 primary node and one or more MySQL 8.0 or later secondaries. The default collation for utf8mb4 in MySQL 5.7 is utf8mb4_general_ci, but utf8mb4_0900_ai_ci in later release series. The variable is not present in releases earlier than MySQL 8.0, so if the replica does not receive a value for the variable, it assumes the source is from an earlier release and sets the value to the previous default collation utf8mb4_general_ci.

The default utf8mb4 collation is used in the following statements:

See also Section 12.9, “Unicode Support”.

default_password_lifetime

This variable defines the global automatic password expiration policy. The default default_password_lifetime value is 0, which disables automatic password expiration. If the value of default_password_lifetime is a positive integer N, it indicates the permitted password lifetime; passwords must be changed every N days.

The global password expiration policy can be overridden as desired for individual accounts using the password expiration option of the CREATE USER and ALTER USER statements. See Section 8.2.15, “Password Management”.

default_storage_engine

The default storage engine for tables. See Chapter 18, Alternative Storage Engines. This variable sets the storage engine for permanent tables only. To set the storage engine for TEMPORARY tables, set the default_tmp_storage_engine system variable.

To see which storage engines are available and enabled, use the SHOW ENGINES statement or query the INFORMATION_SCHEMA ENGINES table.

If you disable the default storage engine at server startup, you must set the default engine for both permanent and TEMPORARY tables to a different engine, or else the server does not start.

default_table_encryption

Defines the default encryption setting applied to schemas and general tablespaces when they are created without specifying an ENCRYPTION clause.

The default_table_encryption variable is only applicable to user-created schemas and general tablespaces. It does not govern encryption of the mysql system tablespace.

Setting the runtime value of default_table_encryption requires the SYSTEM_VARIABLES_ADMIN and TABLE_ENCRYPTION_ADMIN privileges, or the deprecated SUPER privilege.

default_table_encryption supports SET PERSIST and SET PERSIST_ONLY syntax. See Section 7.1.9.3, “Persisted System Variables”.

For more information, see Defining an Encryption Default for Schemas and General Tablespaces.

default_tmp_storage_engine

The default storage engine for TEMPORARY tables (created with CREATE TEMPORARY TABLE). To set the storage engine for permanent tables, set the default_storage_engine system variable. Also see the discussion of that variable regarding possible values.

If you disable the default storage engine at server startup, you must set the default engine for both permanent and TEMPORARY tables to a different engine, or else the server does not start.

default_week_format

The default mode value to use for the WEEK() function. See Section 14.7, “Date and Time Functions”.

delay_key_write

This variable specifies how to use delayed key writes. It applies only to MyISAM tables. Delayed key writing causes key buffers not to be flushed between writes. See also Section 18.2.1, “MyISAM Startup Options”.

This variable 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.

If DELAY_KEY_WRITE is enabled for a table, the key buffer is not flushed for the table on every index update, but only when the table is closed. This speeds 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_options system variable set (for example, myisam_recover_options='BACKUP,FORCE'). See Section 7.1.8, “Server System Variables”, and Section 18.2.1, “MyISAM Startup Options”.

If you start mysqld with --skip-new, delay_key_write is set to OFF.

delayed_insert_limit

This system variable is deprecated (because DELAYED inserts are not supported), and you should expect it to be removed in a future release.

delayed_insert_timeout

This system variable is deprecated (because DELAYED inserts are not supported), and you should expect it to be removed in a future release.

delayed_queue_size

This system variable is deprecated (because DELAYED inserts are not supported), and you should expect it to be removed in a future release.

disabled_storage_engines

This variable indicates which storage engines cannot be used to create tables or tablespaces. For example, to prevent new MyISAM or FEDERATED tables from being created, start the server with these lines in the server option file:

[mysqld]
disabled_storage_engines="MyISAM,FEDERATED"

By default, disabled_storage_engines is empty (no engines disabled), but it can be set to a comma-separated list of one or more engines (not case-sensitive). Any engine named in the value cannot be used to create tables or tablespaces with CREATE TABLE or CREATE TABLESPACE, and cannot be used with ALTER TABLE ... ENGINE or ALTER TABLESPACE ... ENGINE to change the storage engine of existing tables or tablespaces. Attempts to do so result in an ER_DISABLED_STORAGE_ENGINE error.

disabled_storage_engines does not restrict other DDL statements for existing tables, such as CREATE INDEX, TRUNCATE TABLE, ANALYZE TABLE, DROP TABLE, or DROP TABLESPACE. This permits a smooth transition so that existing tables or tablespaces that use a disabled engine can be migrated to a permitted engine by means such as ALTER TABLE ... ENGINE permitted_engine.

It is permitted to set the default_storage_engine or default_tmp_storage_engine system variable to a storage engine that is disabled. This could cause applications to behave erratically or fail, although that might be a useful technique in a development environment for identifying applications that use disabled engines, so that they can be modified.

disabled_storage_engines is disabled and has no effect if the server is started with any of these options: --initialize, --initialize-insecure, --skip-grant-tables.

disconnect_on_expired_password

This variable controls how the server handles clients with expired passwords:

For more information about the interaction of client and server settings relating to expired-password handling, see Section 8.2.16, “Server Handling of Expired Passwords”.

div_precision_increment

This variable indicates the number of digits by which to increase the scale of the result of division operations performed with the / operator. The default value is 4. The minimum and maximum values are 0 and 30, respectively. The following example illustrates the effect of increasing the default value.

mysql> SELECT 1/7;
+--------+
| 1/7    |
+--------+
| 0.1429 |
+--------+
mysql> SET div_precision_increment = 12;
mysql> SELECT 1/7;
+----------------+
| 1/7            |
+----------------+
| 0.142857142857 |
+----------------+

dragnet.log_error_filter_rules

The filter rules that control operation of the log_filter_dragnet error log filter component. If log_filter_dragnet is not installed, dragnet.log_error_filter_rules is unavailable. If log_filter_dragnet is installed but not enabled, changes to dragnet.log_error_filter_rules have no effect.

The effect of the default value is similar to the filtering performed by the log_sink_internal filter with a setting of log_error_verbosity=2.

dragnet.Status status variable can be consulted to determine the result of the most recent assignment to dragnet.log_error_filter_rules.

enterprise_encryption.maximum_rsa_key_size

This variable limits the maximum size of RSA keys generated by MySQL Enterprise Encryption. The variable is available only if the MySQL Enterprise Encryption component component_enterprise_encryption is installed.

The lowest setting is 2048 bits, which is the minimum RSA key length that is acceptable by current best practice. The default setting is 4096 bits. The highest setting is 16384 bits. Generating longer keys can consume significant CPU resources, so you can use this setting to limit keys to a length that provides adequate security for your requirements while balancing this with resource usage. See Section 8.6.2, “Configuring MySQL Enterprise Encryption” for more information.

enterprise_encryption.rsa_support_legacy_padding

This variable controls whether encrypted data and signatures that MySQL Enterprise Encryption produced using the old openssl_udf shared library functions can be decrypted or verified by the MySQL Enterprise Encryption component (component_enterprise_encryption). The variable is available only if the MySQL Enterprise Encryption component is installed.

For the component functions to support decryption and verification for content produced by the old openssl_udf shared library functions, you must set the system variable padding to ON. When ON is set, if the component functions cannot decrypt or verify content when assuming it has the RSAES-OAEP or RSASSA-PSS scheme (as used by the component), they make another attempt assuming it has the RSAES-PKCS1-v1_5 or RSASSA-PKCS1-v1_5 scheme (as used by the openssl_udf shared library functions). When OFF is set, if the component functions cannot decrypt or verify content using their normal schemes, they return null output. See Section 8.6.2, “Configuring MySQL Enterprise Encryption” for more information.

end_markers_in_json

Whether optimizer JSON output should add end markers. See Section 10.15.9, “The end_markers_in_json System Variable”.

eq_range_index_dive_limit

This variable indicates the number of equality ranges in an equality comparison condition when the optimizer should switch from using index dives to index statistics in estimating the number of qualifying rows. It applies to evaluation of expressions that have either of these equivalent forms, where the optimizer uses a nonunique index to look up col_name values:

col_name IN(val1, ..., valN)
col_name = val1 OR ... OR col_name = valN

In both cases, the expression contains N equality ranges. The optimizer can make row estimates using index dives or index statistics. If eq_range_index_dive_limit is greater than 0, the optimizer uses existing index statistics instead of index dives if there are eq_range_index_dive_limit or more equality ranges. Thus, to permit use of index dives for up to N equality ranges, set eq_range_index_dive_limit to N + 1. To disable use of index statistics and always use index dives regardless of N, set eq_range_index_dive_limit to 0.

For more information, see Equality Range Optimization of Many-Valued Comparisons.

To update table index statistics for best estimates, use ANALYZE TABLE.

error_count

The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 15.7.7.19, “SHOW ERRORS Statement”.

event_scheduler

This variable enables or disables, and starts or stops, the Event Scheduler. The possible status values are ON, OFF, and DISABLED. Turning the Event Scheduler OFF is not the same as disabling the Event Scheduler, which requires setting the status to DISABLED. This variable and its effects on the Event Scheduler's operation are discussed in greater detail in Section 27.5.2, “Event Scheduler Configuration”

explain_format

This variable determines the default output format used by EXPLAIN in the absence of a FORMAT option when displaying a query execution plan. Possible values and their effects are listed here:

The setting for this variable also affects EXPLAIN ANALYZE. For this purpose, DEFAULT and TRADITIONAL are interpeted as TREE. If the value of explain_format is JSON and an EXPLAIN ANALYZE statement having no FORMAT option is issued, the statement raises an error (ER_NOT_SUPPORTED_YET).

Using a format specifier with EXPLAIN or EXPLAIN ANALYZE overrides any setting for explain_format.

The explain_format system variable has no effect on EXPLAIN output when this statement is used to display information about table columns.

Setting the session value of explain_format requires no special privileges; setting it on the global level requires SYSTEM_VARIABLES_ADMIN (or the deprecated SUPER privilege). See Section 7.1.9.1, “System Variable Privileges”.

For more information and examples, see Obtaining Execution Plan Information.

explain_json_format_version

Determines the version of the JSON output format used by EXPLAIN FORMAT=JSON statements. Setting this variable to 1 causes the server to use Version 1, which is the linear format used for output from such statements in older versions of MySQL; this is the default in MySQL 9.3. Setting explain_json_format_version to 2 causes the Version 2 format to be used; this JSON output format is based on access paths, and is intended to provide better compatibility with future versions of the MySQL Optimizer. It also includes the JSON format version number in its output.

For an example of use, see Obtaining Execution Plan Information.

explicit_defaults_for_timestamp

This system variable determines whether the server enables certain nonstandard behaviors for default values and NULL-value handling in TIMESTAMP columns. By default, explicit_defaults_for_timestamp is enabled, which disables the nonstandard behaviors. Disabling explicit_defaults_for_timestamp results in a warning.

If explicit_defaults_for_timestamp is disabled, the server enables the nonstandard behaviors and handles TIMESTAMP columns as follows:

The nonstandard behaviors just described are deprecated; expect them to be removed in a future MySQL release.

If explicit_defaults_for_timestamp is enabled, the server disables the nonstandard behaviors and handles TIMESTAMP columns as follows:

If explicit_defaults_for_timestamp is disabled at server startup, this warning appears in the error log:

[Warning] TIMESTAMP with implicit DEFAULT value is deprecated.
Please use --explicit_defaults_for_timestamp server option (see
documentation for more details).

As indicated by the warning, to disable the deprecated nonstandard behaviors, enable the explicit_defaults_for_timestamp system variable at server startup.

For additional information, see Section 13.2.5, “Automatic Initialization and Updating for TIMESTAMP and DATETIME”.

external_user

The external user name used during the authentication process, as set by the plugin used to authenticate the client. With native (built-in) MySQL authentication, or if the plugin does not set the value, this variable is NULL. See Section 8.2.19, “Proxy Users”.

flush

Applies to MyISAM, only.

If ON, the server flushes (synchronizes) 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 synchronizing to disk. See Section B.3.3.3, “What to Do If MySQL Keeps Crashing”. This variable is set to ON if you start mysqld with the --flush option.

flush_time

If this is set to a nonzero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. This option is best used only on systems with minimal resources.

foreign_key_checks

If set to 1 (the default), foreign key constraints are checked. If set to 0, foreign key constraints are ignored, with a couple of exceptions. When re-creating a table that was dropped, an error is returned if the table definition does not conform to the foreign key constraints referencing the table. Likewise, an ALTER TABLE operation returns an error if a foreign key definition is incorrectly formed. For more information, see Section 15.1.22.5, “FOREIGN KEY Constraints”.

Setting this variable has the same effect on NDB tables as it does for InnoDB tables. Typically you leave this setting enabled during normal operation, to enforce referential integrity. Disabling foreign key checking can be useful for reloading InnoDB tables in an order different from that required by their parent/child relationships. See Section 15.1.22.5, “FOREIGN KEY Constraints”.

Setting foreign_key_checks to 0 also affects data definition statements: DROP SCHEMA drops a schema even if it contains tables that have foreign keys that are referred to by tables outside the schema, and DROP TABLE drops tables that have foreign keys that are referred to by other tables.

ft_boolean_syntax

The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 14.9.2, “Boolean Full-Text Searches”.

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

ft_max_word_len

The maximum length of the word to be included in a MyISAM FULLTEXT index.

ft_min_word_len

The minimum length of the word to be included in a MyISAM FULLTEXT index.

ft_query_expansion_limit

The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION.

ft_stopword_file

The file from which to read the list of stopwords for full-text searches on MyISAM tables. The server looks for the file in the data directory unless an absolute path name is given to specify a different directory. 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 storage/myisam/ft_static.c file). Setting this variable to the empty string ('') disables stopword filtering. See also Section 14.9.4, “Full-Text Stopwords”.

general_log

Whether the general query log is enabled. The value can be 0 (or OFF) to disable the log or 1 (or ON) to enable the log. The destination for log output is controlled by the log_output system variable; if that value is NONE, no log entries are written even if the log is enabled.

general_log_file

The name of the general query log file. The default value is host_name.log, but the initial value can be changed with the --general_log_file option.

generated_random_password_length

The maximum number of characters permitted in random passwords generated for CREATE USER, ALTER USER, and SET PASSWORD statements. For more information, see Random Password Generation.

global_connection_memory_limit

Set the total amount of memory that can be used by all user connections; that is, Global_connection_memory should not exceed this amount. Any time that it does, all queries (including any currently running) from regular users are rejected with ER_GLOBAL_CONN_LIMIT.

Memory used by the system users such as the MySQL root user is included in this total, but is not counted towards the disconnection limit; such users are never disconnected due to memory usage.

Memory used by the InnoDB buffer pool is excluded from the total.

You must have the SYSTEM_VARIABLES_ADMIN or SUPER privilege to set this variable.

global_connection_memory_status_limit

This is the maximum amount of memory that can be consumed by all user connections before before Count_hit_query_past_global_connection_memory_status_limit is incremented.

global_connection_memory_tracking

Determines whether the server calculates Global_connection_memory. This variable must be enabled explicitly; otherwise, the memory calculation is not performed, and Global_connection_memory is not set.

You must have the SYSTEM_VARIABLES_ADMIN or SUPER privilege to set this variable.

group_concat_max_len

The maximum permitted result length in bytes for the GROUP_CONCAT() function. The default is 1024.

have_compress

YES if the zlib compression library is available to the server, NO if not. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used.

have_dynamic_loading

YES if mysqld supports dynamic loading of plugins, NO if not. If the value is NO, you cannot use options such as --plugin-load to load plugins at server startup, or the INSTALL PLUGIN statement to load plugins at runtime.

have_geometry

YES if the server supports spatial data types, NO if not.

have_profiling

YES if statement profiling capability is present, NO if not. If present, the profiling system variable controls whether this capability is enabled or disabled. See Section 15.7.7.34, “SHOW PROFILES Statement”.

This variable is deprecated; you should expect it to be removed in a future MySQL release.

have_query_cache

have_query_cache is deprecated, always has a value of NO, and you should expect it to be removed in a future MySQL release.

have_rtree_keys

YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM tables.)

have_statement_timeout

Whether the statement execution timeout feature is available (see Statement Execution Time Optimizer Hints). The value can be NO if the background thread used by this feature could not be initialized.

have_symlink

YES if symbolic link support is enabled, NO if not. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options. If the server is started with the --skip-symbolic-links option, the value is DISABLED.

This variable has no meaning on Windows.

histogram_generation_max_mem_size

The maximum amount of memory available for generating histogram statistics. See Section 10.9.6, “Optimizer Statistics”, and Section 15.7.3.1, “ANALYZE TABLE Statement”.

Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 7.1.9.1, “System Variable Privileges”.

host_cache_size

The MySQL server maintains an in-memory host cache that contains client host name and IP address information and is used to avoid Domain Name System (DNS) lookups; see Section 7.1.12.3, “DNS Lookups and the Host Cache”.

The host_cache_size variable controls the size of the host cache, as well as the size of the Performance Schema host_cache table that exposes the cache contents. Setting host_cache_size has these effects:

The default value is autosized to 128, plus 1 for a value of max_connections up to 500, plus 1 for every increment of 20 over 500 in the max_connections value, capped to a limit of 2000.

hostname

The server sets this variable to the server host name at startup. The maximum length is 255 characters.

identity

This variable is a synonym for the last_insert_id variable. It exists for compatibility with other database systems. You can read its value with SELECT @@identity, and set it using SET identity.

init_connect

A string to be executed by the server for each client that connects. The string consists of one or more SQL statements, separated by semicolon characters.

For users that have the CONNECTION_ADMIN privilege (or the deprecated SUPER privilege), the content of init_connect is not executed. This is done so that an erroneous value for init_connect does not prevent all clients from connecting. For example, the value might contain a statement that has a syntax error, thus causing client connections to fail. Not executing init_connect for users that have the CONNECTION_ADMIN or SUPER privilege enables them to open a connection and fix the init_connect value.

init_connect execution is skipped for any client user with an expired password. This is done because such a user cannot execute arbitrary statements, and thus init_connect execution fails, leaving the client unable to connect. Skipping init_connect execution enables the user to connect and change password.

The server discards any result sets produced by statements in the value of init_connect.

information_schema_stats_expiry

Some INFORMATION_SCHEMA tables contain columns that provide table statistics:

STATISTICS.CARDINALITY
TABLES.AUTO_INCREMENT
TABLES.AVG_ROW_LENGTH
TABLES.CHECKSUM
TABLES.CHECK_TIME
TABLES.CREATE_TIME
TABLES.DATA_FREE
TABLES.DATA_LENGTH
TABLES.INDEX_LENGTH
TABLES.MAX_DATA_LENGTH
TABLES.TABLE_ROWS
TABLES.UPDATE_TIME

Those columns represent dynamic table metadata; that is, information that changes as table contents change.

By default, MySQL retrieves cached values for those columns from the mysql.index_stats and mysql.table_stats dictionary tables when the columns are queried, which is more efficient than retrieving statistics directly from the storage engine. If cached statistics are not available or have expired, MySQL retrieves the latest statistics from the storage engine and caches them in the mysql.index_stats and mysql.table_stats dictionary tables. Subsequent queries retrieve the cached statistics until the cached statistics expire. A server restart or the first opening of the mysql.index_stats and mysql.table_stats tables do not update cached statistics automatically.

The information_schema_stats_expiry session variable defines the period of time before cached statistics expire. The default is 86400 seconds (24 hours), but the time period can be extended to as much as one year.

To update cached values at any time for a given table, use ANALYZE TABLE.

To always retrieve the latest statistics directly from the storage engine and bypass cached values, set information_schema_stats_expiry to 0.

Querying statistics columns does not store or update statistics in the mysql.index_stats and mysql.table_stats dictionary tables under these circumstances:

The statistics cache may be updated during a multiple-statement transaction before it is known whether the transaction commits. As a result, the cache may contain information that does not correspond to a known committed state. This can occur with autocommit=0 or after START TRANSACTION.

information_schema_stats_expiry is a session variable, and each client session can define its own expiration value. Statistics that are retrieved from the storage engine and cached by one session are available to other sessions.

For related information, see Section 10.2.3, “Optimizing INFORMATION_SCHEMA Queries”.

init_file

If specified, this variable names a file containing SQL statements to be read and executed during the startup process. The acceptable format for statements in this file support the following constructs:

If the server is started with the --initialize or --initialize-insecure option, it operates in bootstrap mode and some functionality is unavailable that limits the statements permitted in the file. These include statements that relate to account management (such as CREATE USER or GRANT), replication, and global transaction identifiers. See Section 19.1.3, “Replication with Global Transaction Identifiers”.

Threads created during server startup are used for tasks such as creating the data dictionary, running upgrade procedures, and creating system tables. To ensure a stable and predictable environment, these threads are executed with the server built-in defaults for some system variables, such as sql_mode, character_set_server, collation_server, completion_type, explicit_defaults_for_timestamp, and default_table_encryption.

These threads are also used to execute the statements in any file specified with init_file when starting the server, so such statements execute with the server's built-in default values for those system variables.

innodb_xxx

InnoDB system variables are listed in Section 17.14, “InnoDB Startup Options and System Variables”. These variables control many aspects of storage, memory use, and I/O patterns for InnoDB tables, and are especially important now that InnoDB is the default storage engine.

insert_id

The value to be used by the following INSERT or ALTER TABLE statement when inserting an AUTO_INCREMENT value. This is mainly used with the binary log.

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.

internal_tmp_mem_storage_engine

The storage engine for in-memory internal temporary tables (see Section 10.4.4, “Internal Temporary Table Use in MySQL”). Permitted values are TempTable (the default) and MEMORY.

The optimizer uses the storage engine defined by internal_tmp_mem_storage_engine for in-memory internal temporary tables.

Configuring a session setting for internal_tmp_mem_storage_engine requires the SESSION_VARIABLES_ADMIN or SYSTEM_VARIABLES_ADMIN privilege.

join_buffer_size

The minimum size of the buffer that is used for plain index scans, range index scans, and joins that do not use indexes and thus perform full table scans. This variable also controls the amount of memory used for hash joins. Normally, the best way to get fast joins is to add indexes. Increase the value of join_buffer_size to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.

The default is 256KB. The maximum permissible setting for join_buffer_size is 4GB−1. Larger values are permitted for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB−1 with a warning). The block size is 128, and a value that is not an exact multiple of the block size is rounded down to the next lower multiple of the block size by MySQL Server before storing the value for the system variable. The parser allows values up to the maximum unsigned integer value for the platform (4294967295 or 2³²−1 for a 32-bit system, 18446744073709551615 or 2⁶⁴−1 for a 64-bit system) but the actual maximum is a block size lower.

Unless a Block Nested-Loop or Batched Key Access algorithm is used, there is no gain from setting the buffer larger than required to hold each matching row, and all joins allocate at least the minimum size, so use caution in setting this variable to a large value globally. It is better to keep the global setting small and change the session setting to a larger value only in sessions that are doing large joins, or change the setting on a per-query basis by using a SET_VAR optimizer hint (see Section 10.9.3, “Optimizer Hints”). Memory allocation time can cause substantial performance drops if the global size is larger than needed by most queries that use it.

When Block Nested-Loop is used, a larger join buffer can be beneficial up to the point where all required columns from all rows in the first table are stored in the join buffer. This depends on the query; the optimal size may be smaller than holding all rows from the first tables.

When Batched Key Access is used, the value of join_buffer_size defines how large the batch of keys is in each request to the storage engine. The larger the buffer, the more sequential access is made to the right hand table of a join operation, which can significantly improve performance.

For additional information about join buffering, see Section 10.2.1.7, “Nested-Loop Join Algorithms”. For information about Batched Key Access, see Section 10.2.1.12, “Block Nested-Loop and Batched Key Access Joins”. For information about hash joins, see Section 10.2.1.4, “Hash Join Optimization”.

keep_files_on_create

If a MyISAM table is created with no DATA DIRECTORY option, the .MYD file is created in the database directory. By default, if MyISAM finds an existing .MYD file in this case, it overwrites it. The same applies to .MYI files for tables created with no INDEX DIRECTORY option. To suppress this behavior, set the keep_files_on_create variable to ON (1), in which case MyISAM does not overwrite existing files and returns an error instead. The default value is OFF (0).

If a MyISAM table is created with a DATA DIRECTORY or INDEX DIRECTORY option and an existing .MYD or .MYI file is found, MyISAM always returns an error. It does not overwrite a file in the specified directory.

key_buffer_size

Index blocks for MyISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.

The minimum permissible setting is 0, but you cannot set key_buffer_size to 0 dynamically. A setting of 0 drops the key cache, which is not permitted at runtime. Setting key_buffer_size to 0 is permitted only at startup, in which case the key cache is not initialized. Changing the key_buffer_size setting at runtime from a value of 0 to a permitted non-zero value initializes the key cache.

key_buffer_size can be increased or decreased only in increments or multiples of 4096 bytes. Increasing or decreasing the setting by a nonconforming value produces a warning and truncates the setting to a conforming value.

The maximum permissible setting for key_buffer_size is 4GB−1 on 32-bit platforms. Larger values are permitted for 64-bit platforms. The effective maximum size might be less, depending on your available physical RAM and per-process RAM limits imposed by your operating system or hardware platform. The value of this variable indicates the amount of memory requested. Internally, the server allocates as much memory as possible up to this amount, but the actual allocation might be less.

You can increase the value to get better index handling for all reads and multiple writes; on a system whose primary function is to run MySQL using the MyISAM storage engine, 25% of the machine's total memory is an acceptable value for this variable. However, you should be aware that, if you make the value too large (for example, more than 50% of the machine's total memory), your system might start to page and become extremely slow. This is because MySQL relies on the operating system to perform file system caching for data reads, so you must leave some room for the file system cache. You should also consider the memory requirements of any other storage engines that you may be using in addition to MyISAM.

For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 10.2.5.1, “Optimizing INSERT Statements”.

You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. (See Section 15.7.7, “SHOW Statements”.) The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the DELAY_KEY_WRITE table option.

The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_unused status variable and the buffer block size, which is available from the key_cache_block_size system variable:

1 - ((Key_blocks_unused * key_cache_block_size) / key_buffer_size)

This value is an approximation because some space in the key buffer is allocated internally for administrative structures. Factors that influence the amount of overhead for these structures include block size and pointer size. As block size increases, the percentage of the key buffer lost to overhead tends to decrease. Larger blocks results in a smaller number of read operations (because more keys are obtained per read), but conversely an increase in reads of keys that are not examined (if not all keys in a block are relevant to a query).

It is possible to create multiple MyISAM key caches. The size limit of 4GB applies to each cache individually, not as a group. See Section 10.10.2, “The MyISAM Key Cache”.

key_cache_age_threshold

This value controls the demotion of buffers from the hot sublist of a key cache to the warm sublist. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. See Section 10.10.2, “The MyISAM Key Cache”.

key_cache_block_size

The size in bytes of blocks in the key cache. The default value is 1024. See Section 10.10.2, “The MyISAM Key Cache”.

key_cache_division_limit

The division point between the hot and warm sublists of the key cache buffer list. The value is the percentage of the buffer list to use for the warm sublist. Permissible values range from 1 to 100. The default value is 100. See Section 10.10.2, “The MyISAM Key Cache”.

large_files_support

Whether mysqld was compiled with options for large file support.

large_pages

Whether large page support is enabled (via the --large-pages option). See Section 10.12.3.3, “Enabling Large Page Support”.

large_page_size

If large page support is enabled, this shows the size of memory pages. Large memory pages are supported only on Linux; on other platforms, the value of this variable is always 0. See Section 10.12.3.3, “Enabling Large Page Support”.

last_insert_id

The value to be returned from LAST_INSERT_ID(). This is stored in the binary log when you use LAST_INSERT_ID() in a statement that updates a table. Setting this variable does not update the value returned by the mysql_insert_id() C API function.

lc_messages

The locale to use for error messages. The default is en_US. The server converts the argument to a language name and combines it with the value of lc_messages_dir to produce the location for the error message file. See Section 12.12, “Setting the Error Message Language”.

lc_messages_dir

The directory where error messages are located. The server uses the value together with the value of lc_messages to produce the location for the error message file. See Section 12.12, “Setting the Error Message Language”.

lc_time_names

This variable specifies the locale that controls the language used to display day and month names and abbreviations. This variable affects the output from the DATE_FORMAT(), DAYNAME() and MONTHNAME() functions. Locale names are POSIX-style values such as 'ja_JP' or 'pt_BR'. The default value is 'en_US' regardless of your system's locale setting. For further information, see Section 12.16, “MySQL Server Locale Support”.

license

The type of license the server has.

local_infile

This variable controls server-side LOCAL capability for LOAD DATA statements. Depending on the local_infile setting, the server refuses or permits local data loading by clients that have LOCAL enabled on the client side.

To explicitly cause the server to refuse or permit LOAD DATA LOCAL statements (regardless of how client programs and libraries are configured at build time or runtime), start mysqld with local_infile disabled or enabled, respectively. local_infile can also be set at runtime. For more information, see Section 8.1.6, “Security Considerations for LOAD DATA LOCAL”.

lock_wait_timeout

This variable specifies the timeout in seconds for attempts to acquire metadata locks. The permissible values range from 1 to 31536000 (1 year). The default is 31536000.

This timeout applies to all statements that use metadata locks. These include DML and DDL operations on tables, views, stored procedures, and stored functions, as well as LOCK TABLES, FLUSH TABLES WITH READ LOCK, and HANDLER statements.

This timeout does not apply to implicit accesses to system tables in the mysql database, such as grant tables modified by GRANT or REVOKE statements or table logging statements. The timeout does apply to system tables accessed directly, such as with SELECT or UPDATE.

The timeout value applies separately for each metadata lock attempt. A given statement can require more than one lock, so it is possible for the statement to block for longer than the lock_wait_timeout value before reporting a timeout error. When lock timeout occurs, ER_LOCK_WAIT_TIMEOUT is reported.

lock_wait_timeout also defines the amount of time that a LOCK INSTANCE FOR BACKUP statement waits for a lock before giving up.

locked_in_memory

Whether mysqld was locked in memory with --memlock.

log_error

The default error log destination. If the destination is the console, the value is stderr. Otherwise, the destination is a file and the log_error value is the file name. See Section 7.4.2, “The Error Log”.

log_error_services

The components to enable for error logging. The variable may contain a list with 0, 1, or many elements. In the latter case, elements may be delimited by semicolons or commas, optionally followed by space. A given setting cannot use both semicolon and comma separators. Component order is significant because the server executes components in the order listed.

Any loadable (not built in) component named in log_error_services is implicitly loaded if it is not already loaded. For more information, see Section 7.4.2.1, “Error Log Configuration”.

log_error_suppression_list

The log_error_suppression_list system variable applies to events intended for the error log and specifies which events to suppress when they occur with a priority of WARNING or INFORMATION. For example, if a particular type of warning is considered undesirable “noise” in the error log because it occurs frequently but is not of interest, it can be suppressed. This variable affects filtering performed by the log_filter_internal error log filter component, which is enabled by default (see Section 7.5.3, “Error Log Components”). If log_filter_internal is disabled, log_error_suppression_list has no effect.

The log_error_suppression_list value may be the empty string for no suppression, or a list of one or more comma-separated values indicating the error codes to suppress. Error codes may be specified in symbolic or numeric form. A numeric code may be specified with or without the MY- prefix. Leading zeros in the numeric part are not significant. Examples of permitted code formats:

ER_SERVER_SHUTDOWN_COMPLETE
MY-000031
000031
MY-31
31

Symbolic values are preferable to numeric values for readability and portability. For information about the permitted error symbols and numbers, see MySQL 9.3 Error Message Reference.

The effect of log_error_suppression_list combines with that of log_error_verbosity. For additional information, see Section 7.4.2.5, “Priority-Based Error Log Filtering (log_filter_internal)”.

log_error_verbosity

The log_error_verbosity system variable specifies the verbosity for handling events intended for the error log. This variable affects filtering performed by the log_filter_internal error log filter component, which is enabled by default (see Section 7.5.3, “Error Log Components”). If log_filter_internal is disabled, log_error_verbosity has no effect.

Events intended for the error log have a priority of ERROR, WARNING, or INFORMATION. log_error_verbosity controls verbosity based on which priorities to permit for messages written to the log, as shown in the following table.

There is also a priority of SYSTEM. System messages about non-error situations are printed to the error log regardless of the log_error_verbosity value. These messages include startup and shutdown messages, and some significant changes to settings.

The effect of log_error_verbosity combines with that of log_error_suppression_list. For additional information, see Section 7.4.2.5, “Priority-Based Error Log Filtering (log_filter_internal)”.

log_output

The destination or destinations for general query log and slow query log output. The value is a list one or more comma-separated words chosen from TABLE, FILE, and NONE. TABLE selects logging to the general_log and slow_log tables in the mysql system schema. FILE selects logging to log files. NONE disables logging. If NONE is present in the value, it takes precedence over any other words that are present. TABLE and FILE can both be given to select both log output destinations.

This variable selects log output destinations, but does not enable log output. To do that, enable the general_log and slow_query_log system variables. For FILE logging, the general_log_file and slow_query_log_file system variables determine the log file locations. For more information, see Section 7.4.1, “Selecting General Query Log and Slow Query Log Output Destinations”.

log_queries_not_using_indexes

If you enable this variable with the slow query log enabled, queries that are expected to retrieve all rows are logged. See Section 7.4.5, “The Slow Query Log”. This option does not necessarily mean that no index is used. For example, a query that uses a full index scan uses an index but would be logged because the index would not limit the number of rows.

log_raw

The log_raw system variable is initially set to the value of the --log-raw option. See the description of that option for more information. The system variable may also be set at runtime to change password masking behavior.

log_slow_admin_statements

Include slow administrative statements in the statements written to the slow query log. Administrative statements include ALTER TABLE, ANALYZE TABLE, CHECK TABLE, CREATE INDEX, DROP INDEX, OPTIMIZE TABLE, and REPAIR TABLE.

log_slow_extra

If the slow query log is enabled and the output destination includes FILE, the server writes additional fields to log file lines that provide information about slow statements. See Section 7.4.5, “The Slow Query Log”. TABLE output is unaffected.