最近出現一個很奇怪的MySQL問題，使用不同select語句查詢全部數據集居然得到不同的記錄數。select * 得到4條記錄，select 字段得到的是3條記錄。
具體問題可以看下面的查詢結果：
[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)
通過select uid只得到3行記錄，丟失了其中uid='uid_3'的記錄。本來百思不得其解，后來在同事的提醒下使用了check table，才找到問題的所在。
[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 |
+--------------------+-------+----------+-------------------------------------------------------+
查詢數據不一致的原因是table_myisam的索引文件損壞了，對應的索引文件table_myisam.MYI與數據文件table_myisam.MYD不一致。select *并不需要遍歷每個索引項，只需要獲取第一條記錄，根據鏈表順序訪問，因此當前的索引損壞并沒有影響到select *的使用。而select uid需要遍歷所有索引項，因而只獲取到損壞狀態，三條索引記錄。
解決方案是使用repair table進行表索引的修復。
[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)
修復后使用check table可以看到表狀態變成正常，使用select *與select uid都能獲取到4條記錄。
[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)