MySQL查询在订购时使用filesort尽管索引

Question

修改 我让查询更简单，只是为了测试：

select *
from table1 where date >= '2012-02-02' order by date, col_2 desc

我在日期和col_2上有复合索引，但是当我对我的查询进行解释时，它显示：

+----+-------------+------------------+-------+--------------------------+-----------------+---------+------+------+-----------------------------+
| id | select_type | table            | type  | possible_keys            | key             | key_len | ref  | rows | Extra                       |
+----+-------------+------------------+-------+--------------------------+-----------------+---------+------+------+-----------------------------+
|  1 | SIMPLE      | table1           | range | col_2_date, date         | col_2_date      | 4       | NULL | 4643 | Using where; Using filesort |
+----+-------------+------------------+-------+--------------------------+-----------------+---------+------+------+-----------------------------+

如果我在列col_2和date上有索引，为什么mySQL会使用filesort？如何防止它？

Answer 1

答案是按照您创建索引的顺序订购结果。 如果index是（col_1，col_2），那么请使用... order by col_1 desc, col_2 desc或... order by col_1 asc, col_2 asc而不是... order by col_1 asc, col_2 desc或order by col_2, col_1。