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 SESSION
或 GLOBAL
的系统变量或是用户自定义变量。
语法图
- SetVariableStmt
- Variable
SetVariableStmt ::=
"SET" Variable "=" Expression ("," Variable "=" Expression )*
Variable ::=
("GLOBAL" | "SESSION") SystemVariable
| UserVariable
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)
以 @
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 PERSIST
和SET PERSIST_ONLY
。