关于explain结果的疑问
有两个问题,希望知道的兄弟点拨一下!
+----+-------------+---------------+--------+---------------+---------+---------+--------------+------+----------------------------
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra
+----+-------------+---------------+--------+---------------+---------+---------+--------------+------+----------------------------
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 3 | Using temporary; Using file
| 1 | PRIMARY | T3 | eq_ref | PRIMARY | PRIMARY | 4 | T4.COMPANYID | 1 |
| 1 | PRIMARY | T2 | ref | PRIMARY,2 | PRIMARY | 4 | T4.COMPANYID | 727 | Using where
| 2 | DERIVED | mail_stat_tbl | index | 2 | PRIMARY | 11 | NULL | 2910 |
+----+-------------+---------------+--------+---------------+---------+---------+--------------+------+----------------------------
1、对于最后一行有一个疑问。possible_keys里面只有2,为什么key里面却是PRIMARY?key里面还能出现possible_keys里面没有的?
+----+-------------+---------------+--------+---------------+---------+---------+--------------+------+----------------------------
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra
+----+-------------+---------------+--------+---------------+---------+---------+--------------+------+----------------------------
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 3 | Using temporary; Using file
| 1 | PRIMARY | T3 | eq_ref | PRIMARY | PRIMARY | 4 | T4.COMPANYID | 1 |
| 1 | PRIMARY | T2 | ref | PRIMARY | PRIMARY | 4 | T4.COMPANYID | 3105 | Using where
| 2 | DERIVED | mail_stat_tbl | index | NULL | PRIMARY | 11 | NULL | 6211 | Using where
+----+-------------+---------------+--------+---------------+---------+---------+--------------+------+----------------------------
2、上面是我把2这个index删掉后的结果。rows由2910增加到了6211,这是不是说明2这个index本来还是有用的?