MySQL查询在排序时使用Filesort尽管有索引

Question

Edit: I made query it simpler just to test:

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

I have composite index on date and col_2 however when I do explain on my query it shows:

+----+-------------+------------------+-------+--------------------------+-----------------+---------+------+------+-----------------------------+
| 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 |
+----+-------------+------------------+-------+--------------------------+-----------------+---------+------+------+-----------------------------+

Why does mySQL use filesort if I have index on the columns col_2 and date and how could I prevent it?

Answer 1

答案是按照您创建索引的顺序对结果进行排序。例如，如果索引为（col_1，col_2），则使用... order by col_1 desc, col_2 desc或... order by col_1 asc, col_2 asc和例如，不... order by col_1 asc, col_2 desc order by col_2, col_1或order by col_2, col_1 。