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)
--转自