MySQL查询数据不一致问题_MySQL

MySQL查询数据不一致问题

最近出现一个很奇怪的MySQL问题，使用不同select语句查询全部数据集居然得到不同
的记录数。select * 得到4条记录，select 字段得到的是3条记录。

具体问题可以看下面的查询结果：

[sql]

mysql> select * from table_myisam;

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

| datetime | uid | content | type |

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

| 1 | uid_1 | content_1 | 1 |

| 2 | uid_2 | content_2 | 1 |

| 4 | uid_4 | content_4 | 1 |

| 3 | uid_3 | content_3 | 1 |

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

4 rows in set (0.00 sec)

mysql> select uid from table_myisam;

+-------+

| uid |

+-------+

| uid_1 |

| uid_2 |

| uid_4 |

+-------+

3 rows in set (0.00 sec)

通过select uid只得到3行记录，丢失了其中uid='uid_3'的记录。本来百思不得其解，后来在同事的提醒下使用了check table，才找到问题的所在。

[sql]

mysql> check table table_myisam;

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

| Table | Op | Msg_type | Msg_text |

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

| qitai.table_myisam | check | warning | 1 client is using or hasn't closed the table properly |

| qitai.table_myisam | check | warning | Size of indexfile is: 2049 Should be: 2048 |

| qitai.table_myisam | check | error | Found 3 keys of 4 |

| qitai.table_myisam | check | error | Corrupt |

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

查询数据不一致的原因是table_myisam的索引文件损坏了，对应的索引文件table_myisam.MYI与数据文件table_myisam.MYD不一致。select *并不需要遍历每个索引项，只需要获取第一条记录，根据链表顺序访问，因此当前的索引损坏并没有影响到select *的使用。而select uid需要遍历所有索引项，因而只获取到损坏状态，三条索引记录。

解决方案是使用repair table进行表索引的修复。

[sql]

mysql> repair table table_myisam;

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

| Table | Op | Msg_type | Msg_text |

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

| qitai.table_myisam | repair | status | OK |

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

1 row in set (0.00 sec)

修复后使用check table可以看到表状态变成正常，使用select *与select uid都能获取到4条记录。

[sql]

mysql> check table table_myisam;

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

| Table | Op | Msg_type | Msg_text |

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

| qitai.table_myisam | check | status | OK |

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

1 row in set (0.00 sec)