SET [GLOBAL|SESSION] <variable>

SET [GLOBAL|SESSION] 语句用于对某个 TiDB 的内置变量进行更改。这些变量可以是范围为 SESSIONGLOBAL系统变量或是用户自定义变量

语法图

SetVariableStmt
SETVariable=Expression,Variable=Expression
Variable
GLOBALSESSIONSystemVariableUserVariable

示例

获取 sql_mode 的值:

SHOW GLOBAL VARIABLES LIKE 'sql_mode';
+---------------+-------------------------------------------------------------------------------------------------------------------------------------------+ | Variable_name | Value | +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+ | sql_mode | ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION | +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)
SHOW SESSION VARIABLES LIKE 'sql_mode';
+---------------+-------------------------------------------------------------------------------------------------------------------------------------------+ | Variable_name | Value | +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+ | sql_mode | ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION | +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)

更新全局的 sql_mode

SET GLOBAL sql_mode = 'STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER';
Query OK, 0 rows affected (0.03 sec)

检查更新之后的 sql_mode 的取值,可以看到 SESSION 级别的值没有更新:

SHOW GLOBAL VARIABLES LIKE 'sql_mode';
+---------------+-----------------------------------------+ | Variable_name | Value | +---------------+-----------------------------------------+ | sql_mode | STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER | +---------------+-----------------------------------------+ 1 row in set (0.00 sec)
SHOW SESSION VARIABLES LIKE 'sql_mode';
+---------------+-------------------------------------------------------------------------------------------------------------------------------------------+ | Variable_name | Value | +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+ | sql_mode | ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION | +---------------+-------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)

SET SESSION 则可以立即生效:

SET SESSION sql_mode = 'STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER';
Query OK, 0 rows affected (0.01 sec)
SHOW SESSION VARIABLES LIKE 'sql_mode';
+---------------+-----------------------------------------+ | Variable_name | Value | +---------------+-----------------------------------------+ | sql_mode | STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER | +---------------+-----------------------------------------+ 1 row in set (0.00 sec)

@ 开头的用户变量:

SET @myvar := 5; Query OK, 0 rows affected (0.00 sec) SELECT @myvar, @myvar + 1; +--------+------------+ | @myvar | @myvar + 1 | +--------+------------+ | 5 | 6 | +--------+------------+ 1 row in set (0.00 sec)

MySQL 兼容性

使用 SET [GLOBAL|SESSION] <variable> 更改系统变量上,TiDB 与 MySQL 存在以下差异

  • 与 MySQL 不同,TiDB 中使用 SET GLOBAL 所作的修改会应用于集群中的全部 TiDB 实例。而在 MySQL 中,修改不会应用于副本。
  • TiDB 中的若干变量可读又可设置,这是与 MySQL 相兼容的要求,因为应用程序和连接器常读取 MySQL 变量。例如:JDBC 连接器同时读取和设置缓存查询的参数,尽管并不依赖这一行为。
  • 即使在 TiDB 服务器重启后,SET GLOBAL 的更改也仍然有效。这样,TiDB 中的 SET GLOBAL 更类似于 MySQL 8.0 及更高版本中的 SET PERSIST
  • TiDB 会持久化全局变量,因此 TiDB 不支持 SET PERSISTSET PERSIST_ONLY

另请参阅

文档内容是否有帮助?