SET TRANSACTION

SET TRANSACTION 语句用于在 GLOBAL 或 SESSION 的基础上更改当前的隔离级别，是 SET transaction_isolation ='new-value' 的替代语句，提供 MySQL 和 SQL 标准的兼容性。

语法图

2. SetStmt ::=
3. 'SET' ( VariableAssignmentList |
4. 'PASSWORD' ('FOR' Username)? '=' PasswordOpt |
5. ( 'GLOBAL'| 'SESSION' )? 'TRANSACTION' TransactionChars |
6. 'CONFIG' ( Identifier | stringLit) ConfigItemName EqOrAssignmentEq SetExpr )

8. TransactionChars ::=
9. ( 'ISOLATION' 'LEVEL' IsolationLevel | 'READ' 'WRITE' | 'READ' 'ONLY' AsOfClause? )

11. IsolationLevel ::=
12. ( 'REPEATABLE' 'READ' | 'READ' ( 'COMMITTED' | 'UNCOMMITTED' ) | 'SERIALIZABLE' )

14. AsOfClause ::=
15. ( 'AS' 'OF' 'TIMESTAMP' Expression)

示例

1. SHOW SESSION VARIABLES LIKE 'transaction_isolation';

1. +-----------------------+-----------------+
2. | Variable_name         | Value           |
3. +-----------------------+-----------------+
4. | transaction_isolation | REPEATABLE-READ |
5. +-----------------------+-----------------+
6. 1 row in set (0.00 sec)

1. SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;

1. Query OK, 0 rows affected (0.00 sec)

1. SHOW SESSION VARIABLES LIKE 'transaction_isolation';

1. +-----------------------+----------------+
2. | Variable_name         | Value          |
3. +-----------------------+----------------+
4. | transaction_isolation | READ-COMMITTED |
5. +-----------------------+----------------+
6. 1 row in set (0.01 sec)

1. SET SESSION transaction_isolation = 'REPEATABLE-READ';

1. Query OK, 0 rows affected (0.00 sec)

1. SHOW SESSION VARIABLES LIKE 'transaction_isolation';

1. +-----------------------+-----------------+
2. | Variable_name         | Value           |
3. +-----------------------+-----------------+
4. | transaction_isolation | REPEATABLE-READ |
5. +-----------------------+-----------------+
6. 1 row in set (0.00 sec)

MySQL 兼容性

  • TiDB 支持仅在语法中将事务设置为只读的功能。
  • 不支持隔离级别 READ-UNCOMMITTED 和 SERIALIZABLE。
  • 通过快照隔离 (Snapshot Isolation) 技术，实现乐观事务的 REPEATABLE-READ 隔离级别，和 MySQL 兼容。
  • 在悲观事务中，TiDB 支持与 MySQL 兼容的 REPEATABLE-READ 和 READ-COMMITTED 两种隔离级别。具体描述详见 Isolation Levels。

另请参阅