mysql的四种隔离级别
隔离级别 | 英文 | 说明 | 问题 |
未提交读 | read uncommitted (RU) | A事务已执行，但未提交；B事务查询到A事务的更新后数据；A事务回滚。 | 脏读 |
已提交读 | read committed (RC) | A事务执行更新；B事务查询；A事务又执行更新；B事务再次查询时，前后两次数据不一致。 | 不可重复读 |
可重复读 | repeatable read (RR) | A事务无论执行多少次，只要不提交，B事务查询值都不变；B事务仅查询B事务开始时那一瞬间的数据快照。 | 幻读 |
串行化 | serializable | 不允许读写并发操作，写执行时，读必须等待。 |
默认隔离级别
mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set, 1 warning (0.00 sec)
设置隔离级别
# 设置read uncommitted级别：
mysql> set session transaction isolation level read uncommitted;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@tx_isolation;
+------------------+
| @@tx_isolation |
+------------------+
| READ-UNCOMMITTED |
+------------------+
1 row in set, 1 warning (0.00 sec)
# 设置read committed级别：
mysql> set session transaction isolation level read committed;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set, 1 warning (0.00 sec)
# 设置repeatable read级别：
mysql> set session transaction isolation level repeatable read;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set, 1 warning (0.00 sec)
# 设置serializable级别：
mysql> set session transaction isolation level serializable;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| SERIALIZABLE |
+----------------+
1 row in set, 1 warning (0.00 sec)
