To see the values that a server uses based on its compiled-in defaults and any option files that it reads, use this command:要查看服务器根据其编译的默认值和读取的任何选项文件使用的值，请使用以下命令：

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:要查看服务器仅基于其编译的默认值而忽略任何选项文件中的设置所使用的值，请使用以下命令：

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.要查看正在运行的服务器使用的当前值，请使用SHOW VARIABLES语句或性能模式系统变量表。See Section 27.12.14, “Performance Schema System Variable Tables”.请参阅第27.12.14节，“性能模式系统变量表”。

Section 5.1.9, “Using System Variables”, discusses the syntax for setting and displaying system variable values.第5.1.9节，“使用系统变量”讨论了设置和显示系统变量值的语法。

Section 5.1.9.2, “Dynamic System Variables”, lists the variables that can be set at runtime.第5.1.9.2节，“动态系统变量”列出了可在运行时设置的变量。

Information on tuning system variables can be found in Section 5.1.1, “Configuring the Server”.有关调整系统变量的信息，请参阅第5.1.1节，“配置服务器”。

Section 15.14, “InnoDB Startup Options and System Variables”, lists InnoDB system variables.第15.14节，“InnoDB启动选项和系统变量”列出了InnoDB系统变量。

Section 23.3.3.9.2, “NDB Cluster System Variables”, lists system variables which are specific to NDB Cluster.第23.3.3.9.2节，“NDB集群系统变量”列出了NDB集群特有的系统变量。

For information on server system variables specific to replication, see Section 17.1.6, “Replication and Binary Logging Options and Variables”.有关特定于复制的服务器系统变量的信息，请参阅第17.1.6节，“复制和二进制日志记录选项和变量”。

activate_all_roles_on_login

Whether to enable automatic activation of all granted roles when users log in to the server:当用户登录到服务器时，是否启用所有授权角色的自动激活：

Granted roles include those granted explicitly to the user and those named in the mandatory_roles system variable value.授予的角色包括明确授予用户的角色和在commandory_roles系统变量值中命名的角色。

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. activate_all_roles_on_login仅适用于登录时，以及在定义上下文中执行的存储程序和视图的执行开始时。To change the active roles within a session, use SET ROLE. 要更改会话中的活动角色，请使用SET ROLE。To change the active roles for a stored program, the program body should execute SET ROLE.要更改存储程序的活动角色，程序体应执行SET ROLE。

admin_address

The IP address on which to listen for TCP/IP connections on the administrative network interface (see Section 5.1.12.1, “Connection Interfaces”).在管理网络接口上侦听TCP/IP连接的IP地址（请参阅第5.1.12.1节，“连接接口”）。There is no default admin_address value. If this variable is not specified at startup, the server maintains no administrative interface.没有默认的admin_address值。如果在启动时未指定此变量，则服务器不维护任何管理接口。The server also has a bind_address system variable for configuring regular (nonadministrative) client TCP/IP connections.服务器还有一个bind_address系统变量，用于配置常规（非管理）客户端TCP/IP连接。See Section 5.1.12.1, “Connection Interfaces”.请参阅第5.1.12.1节，“连接接口”。

If admin_address is specified, its value must satisfy these requirements:如果指定了admin_address，则其值必须满足以下要求：

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.IP地址可以指定为IPv4或IPv6地址。如果该值是主机名，则服务器将该名称解析为IP地址并绑定到该地址。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.如果主机名解析为多个IP地址，则服务器使用第一个IPv4地址（如果有），否则使用第一个IPv6地址。

The server treats different types of addresses as follows:服务器处理不同类型的地址如下：

These rules apply to specifying a network namespace for an address:这些规则适用于为地址指定网络命名空间：

For additional information about network namespaces, see Section 5.1.14, “Network Namespace Support”.有关网络命名空间的更多信息，请参阅第5.1.14节，“网络命名空间支持”。

If binding to the address fails, the server produces an error and does not start.如果绑定到地址失败，服务器将产生错误，并且不会启动。

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_address系统变量类似于将服务器绑定到普通客户端连接地址的bind_address系变量，但有以下区别：

admin_port

The TCP/IP port number to use for connections on the administrative network interface (see Section 5.1.12.1, “Connection Interfaces”).用于管理网络接口上连接的TCP/IP端口号（请参阅第5.1.12.1节，“连接接口”）。Setting this variable to 0 causes the default value to be used.将此变量设置为0将使用默认值。

Setting admin_port has no effect if admin_address is not specified because in that case the server maintains no administrative network interface.如果未指定admin_address，则设置admin_port无效，因为在这种情况下，服务器不维护任何管理网络接口。

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. admin_ssl_ca系统变量与ssl_ca类似，只是它应用于管理连接接口而不是主连接接口。For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.有关配置管理接口的加密支持的信息，请参阅加密连接的管理接口支持。

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. admin_ssl_capath系统变量与ssl_capath类似，只是它应用于管理连接接口而不是主连接接口。For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.有关配置管理接口的加密支持的信息，请参阅加密连接的管理接口支持。

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. admin_ssl_cert系统变量与ssl_cert类似，只是它应用于管理连接接口而不是主连接接口。For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.有关配置管理接口的加密支持的信息，请参阅加密连接的管理接口支持。

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. admin_ssl_cipher系统变量与ssl_cipher类似，只是它应用于管理连接接口而不是主连接接口。For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.有关配置管理接口的加密支持的信息，请参阅加密连接的管理接口支持。

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. admin_ssl_crl系统变量与ssl_crl类似，只是它应用于管理连接接口而不是主连接接口。For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.有关配置管理接口的加密支持的信息，请参阅加密连接的管理接口支持。

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. admin_ssl_crlpath系统变量与ssl_crlpath类似，只是它应用于管理连接接口而不是主连接接口。For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.有关配置管理接口的加密支持的信息，请参阅加密连接的管理接口支持。

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. admin_ssl_key系统变量与ssl_key类似，只是它应用于管理连接接口而不是主连接接口。For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.有关配置管理接口的加密支持的信息，请参阅加密连接的管理接口支持。

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. admin_tls_ciphersuites系统变量与tls_cipersuites类似，只是它适用于管理连接接口而不是主连接接口。For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.有关配置管理接口的加密支持的信息，请参阅加密连接的管理接口支持。

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. admin_tls_version系统变量与tls_version类似，只是它应用于管理连接接口而不是主连接接口。For information about configuring encryption support for the administrative interface, see Administrative Interface Support for Encrypted Connections.有关配置管理接口的加密支持的信息，请参阅加密连接的管理接口支持。

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 6.4.1.6, “Windows Pluggable Authentication”.仅当authentication_windows Windows身份验证插件已启用并且调试代码已启用时，此变量才可用。请参阅第6.4.1.6节，“Windows可插拔身份验证”。

This variable sets the logging level for the Windows authentication plugin. The following table shows the permitted values.此变量设置Windows身份验证插件的日志记录级别。下表显示了允许的值。

authentication_windows_use_principal_name

This variable is available only if the authentication_windows Windows authentication plugin is enabled. See Section 6.4.1.6, “Windows Pluggable Authentication”.仅当authentication_windows Windows身份验证插件已启用时，此变量才可用。请参阅第6.4.1.6节，“Windows可插拔身份验证”。

A client that authenticates using the InitSecurityContext() function should provide a string identifying the service to which it connects (targetName).使用InitSecurityContext()函数进行身份验证的客户端应提供一个字符串，用于标识其所连接的服务（targetName）。MySQL uses the principal name (UPN) of the account under which the server is running.MySQL使用运行服务器的帐户的主体名称（UPN）。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.UPN具有以下形式user_id@computer_name并且不需要在任何地方注册即可使用。此UPN由服务器在身份验证握手开始时发送。

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.此变量控一致性务器是否在初始质询中发送UPN。默认情况下，变量处于启用状态。出于安全原因，可以禁用它以避免将服务器的帐户名作为明文发送到客户端。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.如果禁用了该变量，则服务器总是在第一个质询中发送0x00字节，客户端不指定targetName，因此使用NTLM身份验证。

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.如果服务器无法获取其UPN（这种情况主要发生在不支持Kerberos身份验证的环境中），则服务器不会发送UPN，而是使用NTLM身份验证。

autocommit

The autocommit mode. If set to 1, all changes to a table take effect immediately.自动提交模式。如果设置为1，则对表的所有更改都将立即生效。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 13.3.1, “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.

See also Section 25.2.2, “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.此变量控一致性务器是否在数据目录中自动生成SSL密钥和证书文件（如果它们还不存在）。

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, no SSL options other than --ssl are specified, and the server-side SSL files are missing from the data directory. 启动时，如果启用了auto_generate_certs系统变量，未指定--ssl以外的SSL选项，并且数据目录中缺少服务器端SSL文件，则服务器会自动在数据目录中生成服务器端和客户端SSL证书和密钥文件。These files enable secure client connections using SSL; see Section 6.3.1, “Configuring MySQL to Use Encrypted Connections”.这些文件使用SSL实现安全的客户端连接；请参阅第6.3.1节，“配置MySQL以使用加密连接”。

For more information about SSL file autogeneration, including file names and characteristics, see Section 6.3.3.1, “Creating SSL and RSA Certificates and Keys using MySQL”有关SSL文件自动生成的更多信息，包括文件名和特征，请参阅第6.3.3.1节，“使用MySQL创建SSL和RSA证书和密钥”。

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 unencypted connections.sha256_password_auto_generate_rsa_keys和caching_sha2_password_auto_generate_rsa_keys系统变量是相关的，但控制在未加密连接上使用rsa进行安全密码交换所需的rsa密钥对文件的自动生成。

avoid_temporal_upgrade

This variable controls whether ALTER TABLE implicitly upgrades temporal columns found to be in pre-5.6.4 format (TIME, DATETIME, and TIMESTAMP columns without support for fractional seconds precision). Upgrading such columns requires a table rebuild, which prevents any use of fast alterations that might otherwise apply to the operation to be performed.

This variable is disabled by default. Enabling it causes ALTER TABLE not to rebuild temporal columns and thereby be able to take advantage of possible fast alterations.默认情况下，此变量处于禁用状态。启用它会导致ALTER TABLE不重新生成临时列，从而能够利用可能的快速更改。

This variable is deprecated; expect it to be removed in a future MySQL release.此变量已弃用；期望它在未来的MySQL版本中被删除。

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.MySQL可以具有的未完成连接请求数。当MySQL主线程在很短的时间内收到很多连接请求时，这一点就会发挥作用。然后，主线程需要一些时间（尽管很短）来检查连接并启动一个新线程。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.back_log值表示在MySQL暂时停止响应新请求之前的短时间内可以堆叠多少请求。只有当您预计在短时间内会有大量连接时，才需要增加此值。

In other words, this value is the size of the listen queue for incoming TCP/IP connections.换句话说，此值是传入TCP/IP连接的侦听队列的大小。Your operating system has its own limit on the size of this queue.您的操作系统对此队列的大小有自己的限制。The manual page for the Unix listen() system call should have more details.Unix list()系统调用的手册页应该有更多详细信息。Check your OS documentation for the maximum value for this variable.有关此变量的最大值，请查看您的操作系统文档。back_log cannot be set higher than your operating system limit.不能设置得高于您的操作系统限制。

The default value is the value of max_connections, which enables the permitted backlog to adjust to the maximum permitted number of connections.默认值是max_connections的值，它使允许的囤积能够调整到允许的最大连接数。

basedir

The path to the MySQL installation base directory.MySQL安装基本目录的路径。

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 8.4.4, “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.MySQL服务器在一个或多个网络套接字上侦听TCP/IP连接。每个套接字绑定到一个地址，但一个地址可以映射到多个网络接口。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 5.1.12.1, “Connection Interfaces”.

If bind_address is specified, its value must satisfy these requirements:如果指定了bind_address，则其值必须满足以下要求：

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.IP地址可以指定为IPv4或IPv6地址。对于任何主机名值，服务器都会将名称解析为IP地址并绑定到该地址。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.如果主机名解析为多个IP地址，则服务器使用第一个IPv4地址（如果有），否则使用第一个IPv6地址。

The server treats different types of addresses as follows:服务器处理不同类型的地址如下：

These rules apply to specifying a network namespace for an address:这些规则适用于为地址指定网络命名空间：

For additional information about network namespaces, see Section 5.1.14, “Network Namespace Support”.有关网络命名空间的更多信息，请参阅第5.1.14节，“网络命名空间支持”。

If binding to any address fails, the server produces an error and does not start.如果绑定到任何地址失败，服务器将产生错误并且不会启动。

Examples:示例：

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. 当bind_address命名单个值（通配符或非通配符）时，服务器侦听单个套接字，对于通配符地址，该套接字可能绑定到多个网络接口。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. 当bind_address命名多个值的列表时，服务器会侦听每个值一个套接字，每个套接字绑定到一个网络接口。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.套接字的数量与指定的值的数量成线性关系。根据操作系统连接接受效率的不同，长值列表可能会因接受TCP/IP连接而导致性能损失。

Because file descriptors are allocated for listening sockets and network namespace files, it may be necessary to increase the open_files_limit system variable.由于文件描述符是为侦听套接字和网络命名空间文件分配的，因此可能需要增加open_files_limit系统变量。

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.如果您打算将服务器绑定到特定地址，请确保mysql.user系统表包含一个具有管理权限的帐户，您可以使用该帐户连接到该地址。Otherwise, you cannot shut down the server.否则，无法关闭服务器。For example, if you bind the server to *, you can connect to it using all existing accounts.例如，如果将服务器绑定到*，则可以使用所有现有帐户连接到它。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.此变量控制诸如AES之类的基于块的算法的块加密模式。It affects encryption for AES_ENCRYPT() and AES_DECRYPT().它会影响AES_ENCRYPT()和AES_DECRYPT()的加密。

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:例如，此语句使AES加密函数使用256位的密钥长度和CBC模式：

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.尝试将block_encryption_mode设置为包含不支持的密钥长度的值或SSL库不支持的模式时出错。

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.此变量限制缓存树的大小（以每个线程的字节为单位）。Setting it to 0 disables this optimization. The default value is 8MB.将其设置为0将禁用此优化。默认值为8MB。

As of MySQL 8.0.14, 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 5.1.9.1, “System Variable Privileges”.从MySQL 8.0.14开始，设置此系统变量的会话值是一项受限制的操作。会话用户必须具有足够的权限来设置受限制的会话变量。参见第5.1.9.1节，“系统变量权限”。

caching_sha2_password_digest_rounds

The number of hash rounds used by the caching_sha2_password authentication plugin for password storage.caching_sha2_password身份验证插件用于密码存储的哈希回合数。

Increasing the number of hashing rounds above the default value incurs a performance penalty that correlates with the amount of increase:将哈希回合数增加到默认值以上会导致与增加量相关的性能损失：

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.服务器使用此变量来确定是否在数据目录中自动生成RSA私钥/公钥对文件（如果这些文件不存在）。

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.启动时，如果所有这些条件都成立，服务器会自动在数据目录中生成RSA私钥/公钥对文件：sha256_password_auto_generate_RSA_keys或caching_sha2_password_auto_generate_rsa_keys系统变量已启用；没有指定RSA选项；数据目录中缺少RSA文件。These key-pair files enable secure password exchange using RSA over unencrypted connections for accounts authenticated by the sha256_password or caching_sha2_password plugin; see Section 6.4.1.3, “SHA-256 Pluggable Authentication”, and Section 6.4.1.2, “Caching SHA-2 Pluggable Authentication”.这些密钥对文件使通过sha256_password或caching_sha2_password插件验证的帐户能够通过未加密的连接使用RSA进行安全的密码交换；请参阅第6.4.1.3节，“SHA-256可插拔身份验证”和第6.4.1.2节，“缓存SHA-2可插拔身份认证”。

For more information about RSA file autogeneration, including file names and characteristics, see Section 6.3.3.1, “Creating SSL and RSA Certificates and Keys using MySQL”有关RSA文件自动生成的更多信息，包括文件名和特征，请参阅第6.3.3.1节，“使用MySQL创建SSL和RSA证书和密钥”。

The auto_generate_certs system variable is related but controls autogeneration of SSL certificate and key files needed for secure connections using SSL.auto_generate_certs系统变量是相关的，但控制使用SSL进行安全连接所需的SSL证书和密钥文件的自动生成。

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.此变量指定caching_sha2_password身份验证插件的RSA私钥文件的路径名。如果将文件命名为相对路径，则会将其解释为相对于服务器数据目录。文件必须为PEM格式。

For information about caching_sha2_password, see Section 6.4.1.2, “Caching SHA-2 Pluggable Authentication”.有关caching_sha2_password的信息，请参阅第6.4.1.2节，“缓存SHA-2可插拔身份验证”。

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.此变量指定caching_sha2_password身份验证插件的RSA公钥文件的路径名。如果将文件命名为相对路径，则会将其解释为相对于服务器数据目录。文件必须为PEM格式。

For information about caching_sha2_password, including information about how clients request the RSA public key, see Section 6.4.1.2, “Caching SHA-2 Pluggable Authentication”.有关caching_sha2_password的信息，包括有关客户端如何请求RSA公钥的信息，请参阅第6.4.1.2节，“缓存SHA-2可插拔身份验证”。

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.从客户端到达的语句的字符集。此变量的会话值是使用客户端连接到服务器时客户端请求的字符集设置的。(Many clients support a --default-character-set option to enable this character set to be specified explicitly. See also Section 10.4, “Connection Character Sets and Collations”.)（许多客户端支持--default字符集选项，以便显式指定此字符集。另请参阅第10.4节，“连接字符集和排序规则”。）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:当客户端请求的值未知或不可用，或者服务器配置为忽略客户端请求时，变量的全局值用于设置会话值：

Some character sets cannot be used as the client character set.某些字符集不能用作客户端字符集。Attempting to use them as the character_set_client value produces an error.试图将它们用作character_set_client值会产生错误。See Impermissible Client Character Sets.请参见不允许的客户端字符集。

character_set_connection

The character set used for literals specified without a character set introducer and for number-to-string conversion.用于在没有字符集导入器的情况下指定的文字以及用于数字到字符串转换的字符集。For information about introducers, see Section 10.3.8, “Character Set Introducers”.有关介绍人的信息，请参阅第10.3.8节，“字符集介绍人”。

character_set_database

The character set used by the default database. The server sets this variable whenever the default database changes.默认数据库使用的字符集。每当默认数据库发生更改时，服务器都会设置此变量。If there is no default database, the variable has the same value as character_set_server.如果没有默认数据库，则变量的值与character_set_server的值相同。

As of MySQL 8.0.14, setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables.从MySQL 8.0.14开始，设置此系统变量的会话值是一项受限制的操作。会话用户必须具有足够的权限来设置受限制的会话变量。See Section 5.1.9.1, “System Variable Privileges”.请参阅第5.1.9.1节，“系统变量权限”。

The global character_set_database and collation_database system variables are deprecated; expect them to be removed in a future version of MySQL.不赞成使用全局character_set_database和collating_database系统变量；预期它们在MySQL的未来版本中被删除。

Assigning a value to the session character_set_database and collation_database system variables is deprecated and assignments produce a warning.不赞成将值分配给会话character_set_database和colloration_database系统变量，并且分配会产生警告。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.在MySQL的未来版本中，预计会话变量将变为只读（并且对它们的赋值会产生错误），在该版本中，仍然可以访问会话变量来确定默认数据库的数据库字符集和排序规则。

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.文件系统字符集。此变量用于解释引用文件名的字符串文字，例如LOAD DATA和SELECT ... INTO OUTFILE语句和LOAD_FILE()函数。Such file names are converted from character_set_client to character_set_filesystem before the file opening attempt occurs.在尝试打开文件之前，这些文件名会从character_set_client转换为character_set_filesystem。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.默认值为binary，这意味着不会发生转换。对于允许使用多字节文件名的系统，可能更适合使用不同的值。For example, if the system represents file names using UTF-8, set character_set_filesystem to 'utf8mb4'.例如，如果系统使用UTF-8表示文件名，请将character_set_filesystem设置为'utf8mb4'。

As of MySQL 8.0.14, 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 5.1.9.1, “System Variable Privileges”.从MySQL 8.0.14开始，设置此系统变量的会话值是一项受限制的操作。会话用户必须具有足够的权限来设置受限制的会话变量。参见第5.1.9.1节，“系统变量权限”。

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.用于向客户端返回查询结果的字符集。这包括结果数据（如列值）、结果元数据（如列名）和错误消息。

character_set_server

The servers default character set. See Section 10.15, “Character Set Configuration”.服务器的默认字符集。请参阅第10.15节，“字符集配置”。If you set this variable, you should also set collation_server to specify the collation for the character set.如果设置了此变量，还应该设置collification_server来指定字符集的排序规则。

character_set_system

The character set used by the server for storing identifiers. The value is always utf8.服务器用于存储标识符的字符集。该值始终为utf8。

character_sets_dir

The directory where character sets are installed. See Section 10.15, “Character Set Configuration”.安装字符集的目录。请参阅第10.15节，“字符集配置”。

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.一些身份验证插件为自己实现代理用户映射（例如PAM和Windows身份验证插件）。默认情况下，其他身份验证插件不支持代理用户。Of these, some can request that the MySQL server itself map proxy users according to granted proxy privileges: mysql_native_password, sha256_password.其中，有些人可以请求MySQL服务器本身根据授予的代理权限映射代理用户：mysql_native_password、sha256_password。

If the check_proxy_users system variable is enabled, the server performs proxy user mapping for any authentication plugins that make such a request.如果启用了check_proxy_users系统变量，则服务器将为发出此类请求的任何身份验证插件执行代理用户映射。However, it may also be necessary to enable plugin-specific system variables to take advantage of server proxy user mapping support:然而，也可能需要启用特定于插件的系统变量，以利用服务器代理用户映射支持：

For information about user proxying, see Section 6.2.18, “Proxy Users”.有关用户代理的信息，请参阅第6.2.18节，“代理用户”。

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 10.8.4, “Collation Coercibility in Expressions”).

collation_database

The collation used by the default database. The server sets this variable whenever the default database changes.默认数据库使用的排序规则。每当默认数据库发生更改时，服务器都会设置此变量。If there is no default database, the variable has the same value as collation_server.如果没有默认数据库，则该变量的值与collation_server的值相同。

As of MySQL 8.0.18, setting the session value of this system variable is no longer a restricted operation.从MySQL 8.0.18开始，设置此系统变量的会话值不再是一项受限制的操作。

The global character_set_database and collation_database system variables are deprecated; expect them to be removed in a future version of MySQL.不赞成使用全局character_set_database和collating_database系统变量；期望它们在MySQL的未来版本中被删除。

Assigning a value to the session character_set_database and collation_database system variables is deprecated and assignments produce a warning.不赞成将值分配给会话character_set_database和colloration_database系统变量，并且分配会产生警告。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.在MySQL的未来版本中，预计会话变量将变为只读（并且赋值会产生错误），在该版本中，仍然可以访问会话变量来确定默认数据库的数据库字符集和排序规则。

collation_server

The server's default collation. See Section 10.15, “Character Set Configuration”.服务器的默认排序规则。请参阅第10.15节，“字符集配置”。

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.事务处理完成类型。此变量可以采用下表中所示的值。可以使用名称值或相应的整数值来分配变量。

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 13.3.3, “Statements That Cause an Implicit Commit”. It also does not apply for XA COMMIT, XA ROLLBACK, or when autocommit=1.

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.如果为AUTO（默认值），MySQL将允许INSERT和SELECT语句同时运行，用于在数据文件中间没有空闲块的MyISAM表。

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.此变量可以采用下表中所示的值。可以使用名称值或相应的整数值来分配变量。

If you start mysqld with --skip-new, concurrent_insert is set to NEVER.如果您使用--skip-new启动mysqld，那么concurrent_insert将设置为NEVER。

See also Section 8.11.3, “Concurrent Inserts”.另见第8.11.3节，“并发插入”。

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.mysqld服务器在使用Bad handshake进行响应之前等待连接数据包的秒数。默认值为10秒。

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.

core_file

Whether to write a core file if the server unexpectedly exits. This variable is set by the --core-file option.如果服务器意外退出，是否写入核心文件。此变量由--core-file选项设置。

create_admin_listener_thread

Whether to use a dedicated listening thread for client connections on the administrative network interface (see Section 5.1.12.1, “Connection Interfaces”).是否对管理网络接口上的客户端连接使用专用侦听线程（请参阅第5.1.12.1节，“连接接口”）。The default is OFF; that is, the manager thread for ordinary connections on the main interface also handles connections for the administrative interface.默认为OFF；也就是说，主接口上普通连接的管理器线程也处理管理接口的连接。

Depending on factors such as platform type and workload, you may find one setting for this variable yields better performance than the other setting.根据平台类型和工作负载等因素，您可能会发现此变量的一个设置比另一个设置产生更好的性能。

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.如果未指定admin_address，则设置create_admin_listener_thread无效，因为在这种情况下，服务器不维护任何管理网络接口。

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.公共表表达式（CTE）的最大递归深度。服务器终止执行任何重复级别超过此变量值的CTE。For more information, see Limiting Common Table Expression Recursion.有关详细信息，请参阅限制公用表表达式递归。

datadir

The path to the MySQL server data directory. Relative paths are resolved with respect to the current directory.MySQL服务器数据目录的路径。相对路径是相对于当前目录解析的。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.如果您希望服务器自动启动（也就是说，在无法提前知道当前目录的环境中），最好将datadir值指定为绝对路径。

debug

This variable indicates the current debugging settings. It is available only for servers built with debugging support.此变量指示当前调试设置。它仅适用于使用调试支持构建的服务器。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.初始值来自服务器启动时给定的--debug选项实例的值。全局值和会话值可以在运行时设置。

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 5.1.9.1, “System Variable Privileges”.设置此系统变量的会话值是一项受限制的操作。会话用户必须具有足够的权限来设置受限制的会话变量。参见第5.1.9.1节，“系统变量权限”。

Assigning a value that begins with + or - cause the value to added to or subtracted from the current value:指定一个以+或-开头的值会导致该值与当前值相加或相减：

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 5.9.4, “The DBUG Package”.有关更多信息，请参阅第5.9.4节，“DBUG软件包”。

debug_sync

This variable is the user interface to the Debug Sync facility. Use of Debug Sync requires that MySQL be configured with the -DENABLE_DEBUG_SYNC=1 CMake option (see Section 2.9.7, “MySQL Source-Configuration Options”).此变量是调试同步功能的用户界面。使用调试同步需要使用-DENABLE_Debug_Sync=1 选项配置MySQL（请参阅第2.9.7节，“MySQL源配置选项”）。If Debug Sync is not compiled in, this system variable is not available.如果未在中编译调试同步，则此系统变量不可用。

The global variable value is read only and indicates whether the facility is enabled.全局变量值是只读的，指示是否启用了该功能。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.会话值可以由任何用户读取，并且具有与全局变量相同的值。会话值可以设置为控制同步点。

Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables.设置此系统变量的会话值是一项受限制的操作。会话用户必须具有足够的权限来设置受限制的会话变量。See Section 5.1.9.1, “System Variable Privileges”.请参阅第5.1.9.1节，“系统变量权限”。

For a description of the Debug Sync facility and how to use synchronization points, see MySQL Internals: Test Synchronization.有关调试同步功能以及如何使用同步点的描述，请参阅MySQL内部：测试同步。

default_authentication_plugin

The default authentication plugin. These values are permitted:默认的身份验证插件。这些值是允许的：

The default_authentication_plugin value affects these aspects of server operation:default_authentication_plugin值会影响服务器操作的以下方面：

default_collation_for_utf8mb4

For internal use by replication. This system variable is set to the default collation for the utf8mb4 character set.供复制内部使用。此系统变量设置为utf8mb4字符集的默认排序规则。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.变量的值从源复制到副本，以便副本可以正确处理源自具有不同默认排序规则的utf8mb4的源的数据。This variable is primarily intended to support replication from a MySQL 5.7 or older replication source server to a MySQL 8.0 replica server, or group replication with a MySQL 5.7 primary node and one or more MySQL 8.0 secondaries.此变量主要用于支持从MySQL 5.7或更早版本的复制源服务器到MySQL 8.0副本服务器的复制，或使用MySQL 5.7主节点和一个或多个MySQL 8.0辅助节点的组复制。The default collation for utf8mb4 in MySQL 5.7 is utf8mb4_general_ci, but utf8mb4_0900_ai_ci in MySQL 8.0.MySQL 5.7中utf8mb4的默认排序规则是utf8mb0_general_ci，但是在MySQL 8.0中utf8mb4的默认排序规则utf8mf4_0900_ai_ci。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.该变量不存在于MySQL 8.0之前的版本中，因此，如果副本没有接收到该变量的值，则会假定源来自早期版本，并将该值设置为以前的默认排序规则utf8mb4_general_ci。

As of MySQL 8.0.18, setting the session value of this system variable is no longer a restricted operation.从MySQL 8.0.18开始，设置此系统变量的会话值不再是一项受限制的操作。

The default utf8mb4 collation is used in the following statements:以下语句中使用默认的utf8mb4排序规则：

default_password_lifetime

This variable defines the global automatic password expiration policy.此变量定义全局自动密码过期策略。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 6.2.15, “Password Management”.

default_storage_engine

The default storage engine for tables.表的默认存储引擎。See Chapter 16, Alternative Storage Engines. This variable sets the storage engine for permanent tables only.请参阅第16章，“替代存储引擎”。此变量仅为永久表设置存储引擎。To set the storage engine for TEMPORARY tables, set the default_tmp_storage_engine system variable.要为TEMPORARY表设置存储引擎，请设置default_tmp_storage_engine系统变量。

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.如果在服务器启动时禁用默认存储引擎，则必须将永久表和TEMPORARY表的默认引擎设置为不同的引擎，否则服务器不会启动。

default_table_encryption

Defines the default encryption setting applied to schemas and general tablespaces when they are created without specifying an ENCRYPTION clause.定义在不指定ENCRYPTION子句的情况下创建架构和常规表空间时应用于它们的默认加密设置。

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 5.1.9.3, “Persisted System Variables”.

For more information, see Defining an Encryption Default for Schemas and General Tablespaces.有关更多信息，请参阅为架构和常规表空间定义加密默认值。

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 12.7, “Date and Time Functions”.用于WEEK()函数的默认模式值。参见第12.7节，“日期和时间函数”。

delay_key_write

This variable specifies how to use delayed key writes.此变量指定如何使用延迟的密钥写入。It applies only to MyISAM tables. Delayed key writing causes key buffers not to be flushed between writes.它仅适用于MyISAM表。延迟的密钥写入会导致在写入之间不刷新密钥缓冲区。See also Section 16.2.1, “MyISAM Startup Options”.另见第16.2.1节，“MyISAM启动选项”。

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.此变量可以具有以下值之一，以影响可在CREATE TABLE语句中使用的DELAY_KEY_WRITE表选项的处理。

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. 如果为表启用了DELAY_KEY_WRITE，则不会在每次索引更新时为表刷新键缓冲区，而是仅在表关闭时才刷新。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'). 这大大加快了对键的写入速度，但如果使用此功能，则应通过使用myisam_recover_options系统变量集（例如，myisam_recover_options='BACKUP,FORCE'）启动服务器来添加对所有MyISAM表的自动检查。See Section 5.1.8, “Server System Variables”, and Section 16.2.1, “MyISAM Startup Options”.请参阅第5.1.8节，“服务器系统变量”和第16.2.1节，“MyISAM启动选项”。

If you start mysqld with --skip-new, delay_key_write is set to OFF.如果使用--skip-new启动mysqld，delay_key_write将设置为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.此系统变量已弃用（因为不支持DELAYED插入），您应该期望在将来的版本中删除它。

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.此系统变量已弃用（因为不支持DELAYED插入），您应该期望在将来的版本中删除它。

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.此系统变量已弃用（因为不支持DELAYED插入），您应该期望在将来的版本中删除它。

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:此变量指示哪些存储引擎不能用于创建表或表空间。例如，为了防止创建新的MyISAM或FEDERATED表，请在服务器选项文件中使用以下行启动服务器：

[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. 允许将default_storage_engine或default_tmp_storage_engine系统变量设置为禁用的存储引擎。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.这可能会导致应用程序行为不稳定或失败，尽管这在开发环境中可能是一种有用的技术，用于识别使用禁用引擎的应用程序，以便对其进行修改。

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:此变量控一致性务器如何处理密码过期的客户端：

For more information about the interaction of client and server settings relating to expired-password handling, see Section 6.2.16, “Server Handling of Expired Passwords”.有关与过期密码处理相关的客户端和服务器设置交互的更多信息，请参阅第6.2.16节，“过期密码的服务器处理”。

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. 此变量表示用/运算符执行的除法运算的结果的位数。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.默认值为4。最小值和最大值分别为0和30。以下示例说明了增加默认值的效果。

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. 控制log_filter_dragnet错误日志筛选器组件的操作的筛选器规则。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.

As of MySQL 8.0.12, the dragnet.Status status variable can be consulted to determine the result of the most recent assignment to dragnet.log_error_filter_rules.

Prior to MySQL 8.0.12, successful assignments to dragnet.log_error_filter_rules at runtime produce a note confirming the new value:

mysql> SET GLOBAL dragnet.log_error_filter_rules = 'IF prio <> 0 THEN unset prio.';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Note
   Code: 4569
Message: filter configuration accepted:
         SET @@GLOBAL.dragnet.log_error_filter_rules=
         'IF prio!=ERROR THEN unset prio.';

The value displayed by SHOW WARNINGS indicates the “decompiled” canonical representation after the rule set has been successfully parsed and compiled into internal form. SHOW WARNINGS显示的值表示规则集已成功解析并编译为内部形式后的“反编译”规范表示。Semantically, this canonical form is identical to the value assigned to dragnet.log_error_filter_rules, but there may be some differences between the assigned and canonical values, as illustrated by the preceding example:从语义上讲，此规范形式与分配给dragnet.log_error_filter_rules的值相同，但分配的值和规范值之间可能存在一些差异，如前面的示例所示：

For additional information, see Section 5.4.2.4, “Types of Error Log Filtering”, and Section 5.5.3, “Error Log Components”.有关更多信息，请参阅第5.4.2.4节，“错误日志筛选的类型”和第5.5.3节，“错误记录组件”。

end_markers_in_json

Whether optimizer JSON output should add end markers. See MySQL Internals: The end_markers_in_json System Variable.优化器JSON输出是否应添加结束标记。请参阅MySQL内部：end_markers_in_json系统变量。

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. 当优化器在估计符合条件的行数时应从使用索引潜水切换到使用索引统计信息时，此变量指示相等比较条件中相等范围的数量。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:它适用于具有以下等效形式之一的表达式的求值，其中优化器使用非唯一索引来查找col_name值：

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. 在这两种情况下，表达式都包含N个相等范围。优化器可以使用索引潜水或索引统计信息进行行估计。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.要禁用索引统计信息的使用并始终使用索引潜水（无论N），请将eq_range_index_dive_limit设置为0。

For more information, see Equality Range Optimization of Many-Valued Comparisons.有关详细信息，请参阅多值比较的相等范围优化。

To update table index statistics for best estimates, use ANALYZE TABLE.要更新表索引统计信息以获得最佳估计值，请使用ANALYZE TABLE。

error_count

The number of errors that resulted from the last statement that generated messages. 生成消息的最后一条语句所导致的错误数。This variable is read only. See Section 13.7.7.17, “SHOW ERRORS Statement”.此变量是只读的。参见第13.7.7.17节，“SHOW ERRORS语句”。

event_scheduler

This variable enables or disables, and starts or stops, the Event Scheduler. 此变量启用或禁用以及启动或停止事件计划程序。The possible status values are ON, OFF, and DISABLED. 可能的状态值为ON、OFF和DISABLED。Turning the Event Scheduler OFF is not the same as disabling the Event Scheduler, which requires setting the status to DISABLED. 关闭事件计划程序与禁用事件计划程序不同，后者需要将状态设置为DISABLED。This variable and its effects on the Event Scheduler's operation are discussed in greater detail in Section 25.4.2, “Event Scheduler Configuration”在第25.4.2节，“事件调度器配置”中更详细地讨论了此变量及其对事件调度器操作的影响

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. 此系统变量确定服务器是否为TIMESTAMP列中的默认值和NULL值处理启用某些非标准行为。By default, explicit_defaults_for_timestamp is enabled, which disables the nonstandard behaviors. Disabling explicit_defaults_for_timestamp results in a warning.

As of MySQL 8.0.18, setting the session value of this system variable is no longer a restricted operation.从MySQL 8.0.18开始，设置此系统变量的会话值不再是一项受限制的操作。

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.不赞成使用刚才描述的不规范行为；期望它们在未来的MySQL版本中被删除。

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.如警告所示，要禁用不推荐使用的非标准行为，请在服务器启动时启用explicit_defaults_for_timestamp系统变量。

For additional information, see Section 11.2.5, “Automatic Initialization and Updating for TIMESTAMP and DATETIME”.有关更多信息，请参阅第11.2.5节，“TIMESTAMP和DATETIME的自动初始化和更新”。

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. 身份验证过程中使用的外部用户名，由用于验证客户端的插件设置。使用本机（内置）MySQL身份验证，或者如果插件没有设置值，则此变量为NULL。See Section 6.2.18, “Proxy Users”.请参阅第6.2.18节，“代理用户”。

flush

If ON, the server flushes (synchronizes) all changes to disk after each SQL statement. 如果为ON，则服务器会在每次SQL语句之后将所有更改刷新（同步）到磁盘。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”. 通常，MySQL只在每条SQL语句之后将所有更改写入磁盘，并让操作系统处理与磁盘的同步。请参阅B.3.3.3节，“如果MySQL持续崩溃该怎么办”。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.如果设置为非零值，则每隔flush_time秒关闭所有表，以释放资源并将未刷新的数据同步到磁盘。此选项最好仅在资源最少的系统上使用。

foreign_key_checks

If set to 1 (the default), foreign key constraints are checked. 如果设置为1（默认值），则会检查外键约束。If set to 0, foreign key constraints are ignored, with a couple of exceptions. 如果设置为0，则忽略外键约束，但有几个例外。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. 重新创建已删除的表时，如果表定义不符合引用该表的外键约束，则会返回错误。Likewise, an ALTER TABLE operation returns an error if a foreign key definition is incorrectly formed. 同样，如果外键定义的格式不正确，ALTER TABLE操作也会返回错误。For more information, see Section 13.1.20.5, “FOREIGN KEY Constraints”.有关更多信息，请参阅第13.1.20.5节，“FOREIGN KEY约束”。

Setting this variable has the same effect on NDB tables as it does for InnoDB tables. 设置此变量对NDB表的效果与对InnoDB表的效果相同。Typically you leave this setting enabled during normal operation, to enforce referential integrity. 通常，在正常操作期间启用此设置，以强制执行引用完整性。Disabling foreign key checking can be useful for reloading InnoDB tables in an order different from that required by their parent/child relationships. 禁用外键检查对于以不同于父/子关系所需的顺序重新加载InnoDB表非常有用。See Section 13.1.20.5, “FOREIGN KEY Constraints”.请参阅第13.1.20.5节，“FOREIGN KEY约束”。

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 12.10.2, “Boolean Full-Text Searches”.使用IN BOOLEAN MODEE执行的布尔全文搜索支持的运算符列表。参见第12.10.2节，“布尔全文搜索”。

The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:默认变量值为'+ -><()~*:""&|'。更改值的规则如下：

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. 服务器会在数据目录中查找文件，除非提供了一个绝对路径名来指定不同的目录。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 12.10.4, “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.是否启用了常规查询日志。该值可以为0（或OFF）以禁用日志，也可以为1（或ON）以启用日志。日志输出的目的地由log_output系统变量控制；如果该值为NONE，则即使启用了日志，也不会写入任何日志条目。

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.有关详细信息，请参阅随机密码生成。

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_openssl

This variable is a synonym for have_ssl.此变量是have_ssl的同义词。

As of MySQL 8.0.26, have_openssl is deprecated and subject to removal in a future MySQL version. 从MySQL 8.0.26开始，have_openssl已被弃用，并将在未来的MySQL版本中删除。For information about TLS properties of MySQL connection interfaces, use the tls_channel_status table.有关MySQL连接接口的TLS属性的信息，请使用tls_channel_status表。

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 13.7.7.31, “SHOW PROFILES Statement”.

This variable is deprecated and you should expect it to be removed in a future MySQL release.此变量已被弃用，您应该期待它在未来的MySQL版本中被删除。

have_query_cache

The query cache was removed in MySQL 8.0.3. have_query_cache is deprecated, always has a value of NO, and you should expect it to be removed in a future MySQL release.MySQL 8.0.3中删除了查询缓存。have_query_cache不推荐使用，它的值总是NO，您应该预期它在未来的MySQL版本中被删除。

have_rtree_keys

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

have_ssl

YES if mysqld supports SSL connections, DISABLED if the server was compiled with SSL support, but was not started with the appropriate connection-encryption options. For more information, see Section 2.9.6, “Configuring SSL Library Support”.

As of MySQL 8.0.26, have_ssl is deprecated and subject to removal in a future MySQL version. For information about TLS properties of MySQL connection interfaces, use the tls_channel_status table.

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.语句执行超时功能是否可用（请参阅语句执行时间优化器提示）。如果此功能使用的后台线程无法初始化，则该值可以为NO。

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. 可用于生成直方图统计信息的最大内存量。See Section 8.9.6, “Optimizer Statistics”, and Section 13.7.3.1, “ANALYZE TABLE Statement”.请参阅第8.9.6节，“优化器统计”和第13.7.3.1节，“ANALYZE TABLE语句”。

Setting the session value of this system variable is a restricted operation. 设置此系统变量的会话值是一项受限制的操作。The session user must have privileges sufficient to set restricted session variables. 会话用户必须具有足够的权限来设置受限制的会话变量。See Section 5.1.9.1, “System Variable Privileges”.请参阅第5.1.9.1节，“系统变量权限”。

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 5.1.12.3, “DNS Lookups and the Host Cache”.MySQL服务器维护一个内存中的主机缓存，该缓存包含客户端主机名和IP地址信息，用于避免域名系统（DNS）查找；请参阅第5.1.12.3节，“DNS查找和主机缓存”。

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. host_cache_size变量控制主机缓存的大小，以及公开缓存内容的性能架构host_cache表的大小。Setting host_cache_size has these effects:设置host_cache_size具有以下效果：

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.

Using the --skip-host-cache option is similar to setting the host_cache_size system variable to 0, but host_cache_size is more flexible because it can also be used to resize, enable, and disable the host cache at runtime, not just at server startup. Starting the server with --skip-host-cache does not prevent runtime changes to the value of host_cache_size, but such changes have no effect and the cache is not re-enabled even if host_cache_size is set larger than 0.

hostname

The server sets this variable to the server host name at startup. The maximum length is 255 characters as of MySQL 8.0.17, per RFC 1034, and 60 characters before that.服务器在启动时将此变量设置为服务器主机名。根据RFC 1034，从MySQL 8.0.17开始，最大长度为255个字符，在此之前为60个字符。

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.由服务器为每个连接的客户端执行的字符串。该字符串由一个或多个SQL语句组成，用分号分隔。

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.服务器将丢弃init_connect值中的语句生成的任何结果集。

information_schema_stats_expiry

Some INFORMATION_SCHEMA tables contain columns that provide table statistics:某些INFORMATION_SCHEMA表包含提供表统计信息的列：

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.这些列表示动态表元数据；即随着表内容的变化而变化的信息。

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.

The information_schema_stats_expiry session variable defines the period of time before cached statistics expire. information_schema_stats_experity会话变量定义缓存统计信息过期前的一段时间。The default is 86400 seconds (24 hours), but the time period can be extended to as much as one year.默认值为86400秒（24小时），但时间段可以延长到一年。

To update cached values at any time for a given table, use ANALYZE TABLE.要随时更新给定表的缓存值，请使用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. 在知道事务是否提交之前，可以在多语句事务期间更新统计缓存。As a result, the cache may contain information that does not correspond to a known committed state. 结果，高速缓存可能包含与已知提交状态不对应的信息。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. information_schema_stas_express是一个会话变量，每个客户端会话都可以定义自己的过期值。Statistics that are retrieved from the storage engine and cached by one session are available to other sessions.从存储引擎中检索并由一个会话缓存的统计信息可用于其他会话。

For related information, see Section 8.2.3, “Optimizing INFORMATION_SCHEMA Queries”.有关相关信息，请参阅第8.2.3节，“优化INFORMATION_SCHEMA查询”。

init_file

If specified, this variable names a file containing SQL statements to be read and executed during the startup process. 如果指定，此变量将命名一个文件，该文件包含要在启动过程中读取和执行的SQL语句。Prior to MySQL 8.0.18, each statement must be on a single line and should not include comments. As of MySQL 8.0.18, the acceptable format for statements in the file is expanded to support these constructs:在MySQL 8.0.18之前，每条语句必须在一行上，并且不应包含注释。从MySQL 8.0.18开始，文件中语句的可接受格式被扩展为支持以下结构：

If the server is started with the --initialize or --initialize-insecure option, it operates in bootstap 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 17.1.3, “Replication with Global Transaction Identifiers”.

As of MySQL 8.0.17, 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 15.14, “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. 服务器在关闭交互式连接之前等待活动的秒数。An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). 交互式客户端定义为对mysql_real_connect()使用CLIENT_INTERACTIVE选项的客户端。See also wait_timeout.另请参见wait_timeout。

internal_tmp_disk_storage_engine

Prior to MySQL 8.0.16, this variable determines the storage engine used for on-disk internal temporary tables (see Storage Engine for On-Disk Internal Temporary Tables). 在MySQL 8.0.16之前，此变量确定用于磁盘上内部临时表的存储引擎（请参阅磁盘上内部暂时表的存储发动机）。Permitted values are MYISAM and INNODB (the default).允许的值为MYISAM和InnoDB（默认值）。

internal_tmp_mem_storage_engine

The storage engine for in-memory internal temporary tables (see Section 8.4.4, “Internal Temporary Table Use in MySQL”). 内存中内部临时表的存储引擎（请参阅第8.4.4节，“MySQL中的内部临时表使用”）。Permitted values are TempTable (the default) and MEMORY.允许的值为TempTable（默认值）和MEMORY。

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

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. Normally, the best way to get fast joins is to add indexes. 用于纯索引扫描、范围索引扫描以及不使用索引并因此执行全表扫描的联接的缓冲区的最小大小。通常，获得快速联接的最佳方法是添加索引。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. 当无法添加索引时，增加join_buffer_size的值以获得更快的完全联接。为两个表之间的每个完整联接分配一个联接缓冲区。For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.对于不使用索引的多个表之间的复杂联接，可能需要多个联接缓冲区。

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. 除非使用块嵌套循环或批处理密钥访问算法，否则将缓冲区设置为大于容纳每个匹配行所需的缓冲区不会带来任何好处，并且所有联接都至少分配最小大小，因此在全局范围内将此变量设置为大值时要小心。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 8.9.3, “Optimizer Hints”). 最好保持全局设置较小，并仅在执行大型联接的会话中将会话设置更改为较大值，或者使用SET_VAR优化器提示按每个查询更改设置（请参见第8.9.3节，“优化器提示”）。Memory allocation time can cause substantial performance drops if the global size is larger than needed by most queries that use it.如果全局大小大于大多数使用内存分配时间的查询所需的大小，那么内存分配时间可能会导致性能大幅下降。

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.当使用块嵌套循环时，更大的联接缓冲区可能是有益的，直到第一个表中所有行的所有必需列都存储在联接缓冲区中。这取决于查询；最佳大小可以小于保持来自第一表的所有行。

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.

The default is 256KB. The maximum permissible setting for join_buffer_size is 4GB−1. 默认值为256KB。join_buffer_size的最大允许设置为4GB1。Larger values are permitted for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB−1 with a warning).64位平台允许使用较大的值（64位Windows除外，对于64位Windows，较大的值会截断为4GB1并发出警告）。

For additional information about join buffering, see Section 8.2.1.7, “Nested-Loop Join Algorithms”. For information about Batched Key Access, see Section 8.2.1.12, “Block Nested-Loop and Batched Key Access Joins”.有关连接缓冲的更多信息，请参阅第8.2.1.7节，“嵌套循环连接算法”。有关批量密钥访问的信息，请参阅第8.2.1.12节，“块嵌套循环和批量密钥访问连接”。

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.它不会覆盖指定目录中的文件。

key_buffer_size

Index blocks for MyISAM tables are buffered and are shared by all threads. MyISAM表的索引块是缓冲的，并由所有线程共享。key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.key_buffer_size是用于索引块的缓冲区的大小。密钥缓冲区也称为密钥缓存。

The maximum permissible setting for key_buffer_size is 4GB−1 on 32-bit platforms. Larger values are permitted for 64-bit platforms. 在32位平台上，key_buffer_size的最大允许设置为4GB-1。64位平台允许使用更大的值。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. 有效的最大大小可能会更小，这取决于您的可用物理RAM和操作系统或硬件平台对每个进程的RAM限制。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.此变量的值表示请求的内存量。在内部，服务器会分配尽可能多的内存，但实际分配的内存可能会更少。

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. 您可以增加该值以更好地处理所有读取和多次写入的索引；在一个主要功能是使用MyISAM存储引擎运行MySQL的系统上，该变量可以接受机器总内存的25%。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. 但是，您应该注意，如果您使该值过大（例如，超过机器总内存的50%），系统可能会开始分页并变得非常缓慢。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. 这是因为MySQL依赖于操作系统来执行数据读取的文件系统缓存，所以必须为文件系统缓存留出一些空间。You should also consider the memory requirements of any other storage engines that you may be using in addition to MyISAM.除了MyISAM之外，您还应该考虑可能使用的任何其他存储引擎的内存需求。

For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 8.2.5.1, “Optimizing INSERT Statements”.为了在同时写入多行时获得更高的速度，请使用LOCK TABLES。参见第8.2.5.1节，“优化INSERT语句”。

You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. (See Section 13.7.7, “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.如果您主要使用更新和删除，Key_writes/Key_write_requests的比率通常接近1，但如果您倾向于同时进行影响多行的更新，或者如果您使用DELAY_KEY_WRITE表选项，则比率可能会小得多。

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. 这个值是近似值，因为密钥缓冲区中的一些空间是在内部分配给管理结构的。影响这些结构的开销量的因素包括块大小和游标大小。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).随着块大小的增加，由于开销而丢失的密钥缓冲区的百分比往往会降低。较大的块会导致较少的读取操作次数（因为每次读取会获得更多的密钥），但相反，会增加未检查的密钥的读取次数（如果不是块中的所有密钥都与查询相关）。

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 8.10.2, “The MyISAM Key Cache”.可以创建多个MyISAM密钥缓存。4GB的大小限制单独应用于每个缓存，而不是作为一个组。请参阅第8.10.2节，“MyISAM密钥缓存”。

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. 此值控制缓冲区从密钥缓存的热子列表降级到热子列表。值越低，降级发生得越快。The minimum value is 100. The default value is 300. See Section 8.10.2, “The MyISAM Key Cache”.最小值为100。默认值为300。请参阅第8.10.2节，“MyISAM密钥缓存”。

key_cache_block_size

The size in bytes of blocks in the key cache. The default value is 1024. 密钥缓存中块的大小（以字节为单位）。默认值为1024。See Section 8.10.2, “The MyISAM Key Cache”.请参阅第8.10.2节，“MyISAM密钥缓存”。

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. 密钥缓存缓冲区列表的热子列表和热子列表之间的分界点。该值是用于暖子列表的缓冲区列表的百分比。允许值的范围从1到100。The default value is 100. See Section 8.10.2, “The MyISAM Key Cache”.默认值为100。请参阅第8.10.2节，“MyISAM密钥缓存”。

large_files_support

Whether mysqld was compiled with options for large file support.是否使用支持大文件的选项编译了mysqld。

large_pages