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
- SetStmt
- TransactionChars
- IsolationLevel
- AsOfClause
SetStmt ::=
'SET' ( VariableAssignmentList |
'PASSWORD' ('FOR' Username)? '=' PasswordOpt |
( 'GLOBAL'| 'SESSION' )? 'TRANSACTION' TransactionChars |
'CONFIG' ( Identifier | stringLit) ConfigItemName EqOrAssignmentEq SetExpr )
TransactionChars ::=
( 'ISOLATION' 'LEVEL' IsolationLevel | 'READ' 'WRITE' | 'READ' 'ONLY' AsOfClause? )
IsolationLevel ::=
( 'REPEATABLE' 'READ' | 'READ' ( 'COMMITTED' | 'UNCOMMITTED' ) | 'SERIALIZABLE' )
AsOfClause ::=
( 'AS' 'OF' 'TIMESTAMP' Expression)
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。