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)