mysql索引为何没有用到

Question

mysql> explain select * from T_Alarm_Information where recover_Date>'2013-06-03 09:57:00' and creat_Date<'2013-06-03 11:57:00';
+----+-------------+---------------------+------+------------------------+------+---------+------+----------+-------------+
| id | select_type | table               | type | possible_keys          | key  | key_len | ref  | rows     | Extra       |
+----+-------------+---------------------+------+------------------------+------+---------+------+----------+-------------+
|  1 | SIMPLE      | T_Alarm_Information | ALL  | i_t_alarm_info_rectime | NULL | NULL    | NULL | 15647542 | Using where |
+----+-------------+---------------------+------+------------------------+------+---------+------+----------+-------------+
1 row in set (0.00 sec)但是我的索引是存在的啊
| t_alarm_information |          1 | i_t_alarm_info_rectime                       |            1 | CREAT_DATE       | A         |      205888 |     NULL | NULL   |      | BTREE      |         |
| t_alarm_information |          1 | i_t_alarm_info_rectime                       |            2 | RECOVER_DATE     | A         |      340163 |     NULL | NULL   | YES  | BTREE   
为什么这里会用不到索引呢。表里有1500W数据，查询得到的结果是91条

Answer 1

应该是MYSQL通过索引判断，表中几乎所有记录都符合 creat_Date<'2013-06-03 11:57:00' 条件，这种情况下，使用索引效率反而更差。先创建一个 RECOVER_DATE的索引试一下。

Answer 2

谢谢了，试了下果然OK了。谢谢ACMAIN_CHM版主