Question

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

SetStmt:

TransactionChar:

IsolationLevel:

示例

SHOW SESSION VARIABLES LIKE 'transaction_isolation';

+-----------------------+-----------------+
| Variable_name         | Value           |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;

Query OK, 0 rows affected (0.00 sec)

SHOW SESSION VARIABLES LIKE 'transaction_isolation';

+-----------------------+----------------+
| Variable_name         | Value          |
+-----------------------+----------------+
| transaction_isolation | READ-COMMITTED |
+-----------------------+----------------+
1 row in set (0.01 sec)

SET SESSION transaction_isolation = 'REPEATABLE-READ';

Query OK, 0 rows affected (0.00 sec)

SHOW SESSION VARIABLES LIKE 'transaction_isolation';

+-----------------------+-----------------+
| Variable_name         | Value           |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
+-----------------------+-----------------+
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

• SET \[GLOBAL|SESSION\] <variable>
• Isolation Levels