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:
    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.
    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。
    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.
    mysql> check table table_myisam;
    +--------------------+-------+----------+----------+
    | Table              | Op    | Msg_type | Msg_text |
    +--------------------+-------+----------+----------+
    | qitai.table_myisam | check | status   | OK       |
    +--------------------+-------+----------+----------+
    1 row in set (0.00 sec)