mysql怪异问题 对于个别值查询慢

Question

有两个表所有的查询速度都很快，只有cid＝5的情况下速度会很慢，重建索引也没见效果；
这是什么问题导致的。存在着歌问题貌似有段时间了，count了下数量，cid＝5的条数最少，查询时间最耗时；

mysql> SELECT `uuid` FROM `ey_article` WHERE `cid` = 4 ORDER BY kid DESC LIMIT 0,1;
+----------------------------------+
| uuid                               |
+----------------------------------+
| 62ff694813f3c564484d864d977bfd50 |
+----------------------------------+
1 row in set (0.01 sec)

mysql> SELECT `uuid` FROM `ey_article` WHERE `cid` = 1 ORDER BY kid DESC LIMIT 0,1;
+----------------------------------+
| uuid                               |
+----------------------------------+
| 1246f583e18537d10fbd6c65dd258396 |
+----------------------------------+
1 row in set (0.00 sec)

mysql> SELECT `uuid` FROM `ey_article` WHERE `cid` = 2 ORDER BY kid DESC LIMIT 0,1;
+----------------------------------+
| uuid                               |
+----------------------------------+
| d73421575ca1af800df8dedf200d8da4 |
+----------------------------------+
1 row in set (0.00 sec)

mysql> SELECT `uuid` FROM `ey_article` WHERE `cid` = 5 ORDER BY kid DESC LIMIT 0,1;
+----------------------------------+
| uuid                               |
+----------------------------------+
| b32d55762b848758c885b7a5c1a027cf |
+----------------------------------+
1 row in set (2.61 sec)