刷脉脉,发现一个帖子讨论幻读问题:

http://maimai.cn/web/gossip_detail?src=app&webid=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJlZ2lkIjoiN2JmMjA4ZDZjNzU0MTFlYWExOTk4MDE4NDRlNTAxOTAiLCJ1IjoyMjM0MjgzMTksImlkIjoyNjc0NDU3OH0.yvtEe5Z1vjtmPE5dBwXVqD-pMWBE2--jDQuRARcWArI


固做一下实验,演示下MySQL默认隔离级别REPEATABLE-READ并没有解决幻读问题


幻读演示 

MySQL默认隔离级别REPEATABLE-READ(可重复读)

会话一

会话二

MySQL [test]> select * from t1;

+------+

| id   |

+------+

|    1 |

|    2 |

|    3 |

|    4 |

+------+

4 rows in set (0.000 sec)

MySQL [test]> select * from t1;

+------+

| id   |

+------+

|    1 |

|    2 |

|    3 |

|    4 |

+------+

4 rows in set (0.000 sec)

MySQL [test]> begin;

Query OK, 0 rows affected (0.000 sec)

 

注:开启事务一

MySQL [test]> begin;

Query OK, 0 rows affected (0.000 sec)

 

注:开启事务二

MySQL [test]> insert into t1 values(5);

Query OK, 1 row affected (0.000 sec)

 

MySQL [test]> select * from t1;

+------+

| id   |

+------+

|    1 |

|    2 |

|    3 |

|    4 |

|    5 |

+------+

5 rows in set (0.000 sec)

 

注:插入一条数据5

MySQL [test]> select * from t1;

+------+

| id   |

+------+

|    1 |

|    2 |

|    3 |

|    4 |

+------+

4 rows in set (0.000 sec)

 

注:因会话一未提交,所以在会话二事务里是看不见更改后的结果的

MySQL [test]> commit;

Query OK, 0 rows affected (0.002 sec)

 

注:会话一执行事务提交



MySQL [test]> select * from t1;

+------+

| id   |

+------+

|    1 |

|    2 |

|    3 |

|    4 |

+------+

4 rows in set (0.000 sec)

 

MySQL [test]> update t1 set id = id+10;

Query OK, 5 rows affected (0.001 sec)

Rows matched: 5  Changed: 5  

Warnings: 0

 

注:执行全表更新,id+10


MySQL [test]> select * from t1;

+------+

| id   |

+------+

|   11 |

|   12 |

|   13 |

|   14 |

|   15 |

+------+

5 rows in set (0.000 sec)

 

注:当再次查看时,此时发现有5条数据被更改,产生幻读


MySQL [test]> select version();

+-----------+

| version() |

+-----------+

| 8.0.21    |

+-----------+

1 row in set (0.000 sec)

 

MySQL默认隔离级Repeatable Read下,刚才的操作,在会话二未提交的事务里,

会莫名其妙地看到第5条数据,这种现象称为幻读。


固只能通过select ... for update才能避免幻读。