mysql-事务-校验不可重复读和幻象读

mysql-事务-校验不可重复读和幻象读

问题描述

为了验证mysql在默念的隔离级别是不是解决了不可重复读和幻象读的问题

预备

• 不可重复读: 事务A读到了事务B已经提交的修改数据
• 幻象读: 事务A读到了事务B已经提交的新增数据

校验

查看mysql数据库默认隔离级别

mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set

验证是否解决不可重复读

• 事务A查询用户金额,此时开启事务B
• 事务B修改用户的金额，并提交事务
• 查看事务A中是否读取到事务B提交的修改

事务A                           |    事务B
mysql> start transaction;       |    mysql> start transaction;
Query OK, 0 rows affected       |    Query OK, 0 rows affected
                                |    
mysql> select * from t_account; |    mysql> select * from t_account;
+----+---------+--------+       |    +----+---------+--------+
| id | user_id | amount |       |    | id | user_id | amount |
+----+---------+--------+       |    +----+---------+--------+
|  1 |       1 |     23 |       |    |  1 |       1 |     23 |
+----+---------+--------+       |    +----+---------+--------+
1 row in set                    |    1 row in set
                                |    
                                |    mysql> update t_account set amount=10 where user_id=1;
                                |    Query OK, 1 row affected
                                |    Rows matched: 1  Changed: 1  Warnings: 0
                                |    mysql> select * from t_account;
                                |    +----+---------+--------+
                                |    | id | user_id | amount |
                                |    +----+---------+--------+
                                |    |  1 |       1 |     10 |
                                |    +----+---------+--------+
                                |    1 row in set
                                |    # 事务B提交修改
                                |    mysql> commit;
                                |    Query OK, 0 rows affected
mysql> select * from t_account; |
+----+---------+--------+       |
| id | user_id | amount |       |
+----+---------+--------+       |
|  1 |       1 |     23 |       |
+----+---------+--------+       |
1 row in set                    |


# 事务A也提交
mysql> commit;
Query OK, 0 rows affected

mysql> select * from t_account;
+----+---------+--------+
| id | user_id | amount |
+----+---------+--------+
|  1 |       1 |     10 |
+----+---------+--------+
1 row in set

可以发现，直至事务B提交了事务，在事务A中，也没有读取到事务B修改的值。只有事务A也提交事务，才能查询到修改的值，但这已经没有事务了

验证是否解决幻象读

• 事务A查询用户金额,此时开启事务B
• 事务B新增另一个用户的帐户，并提交事务
• 查看事务A中是否读取到事务B提交的新增用户的帐户的数据

事务A                           | 事务B
mysql> start transaction;       | mysql> start transaction;
Query OK, 0 rows affected       | Query OK, 0 rows affected
                                | 
mysql> select * from t_account; | mysql> select * from t_account;
+----+---------+--------+       | +----+---------+--------+
| id | user_id | amount |       | | id | user_id | amount |
+----+---------+--------+       | +----+---------+--------+
|  1 |       1 |     10 |       | |  1 |       1 |     10 |
+----+---------+--------+       | +----+---------+--------+
1 row in set                    | 1 row in set
                                | 
                                | mysql> INSERT INTO `dbl`.`t_account` (`user_id`, `amount`)
								|								VALUES ('2', '8');
                                | 
                                | Query OK, 1 row affected
                                | mysql> select * from t_account;
                                | +----+---------+--------+
                                | | id | user_id | amount |
                                | +----+---------+--------+
                                | |  1 |       1 |     10 |
                                | |  3 |       2 |      8 |
                                | +----+---------+--------+
                                | 2 rows in set
                                | 
                                | mysql> commit;
                                | Query OK, 0 rows affected
mysql> select * from t_account; |
+----+---------+--------+       |
| id | user_id | amount |       |
+----+---------+--------+       |
|  1 |       1 |     10 |       |
+----+---------+--------+       |
1 row in set                    |

可以发现，直至事务B提交了事务，在事务A中，也没有读取到事务B新增的帐户数据

结论

由上述验证，发现，mysql默认的隔离级别(repeatable read)确实解决了不可重复读和幻象读的问题