mysql查询数据不一致原因分析
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)