Mysql Explain 詳解

一.語(yǔ)法

explain < table_name >

例如: explain select * from t3 where id=3952602;

二.explain輸出解釋

+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+
| id | select_type | table | type??| possible_keys? ???| key? ???| key_len | ref? ?| rows | Extra |
+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+

1.id
??我的理解是SQL執(zhí)行的順利的標(biāo)識(shí),SQL從大到小的執(zhí)行.

例如:
mysql> explain select * from (select * from ( select * from t3 where id=3952602) a) b;
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| id | select_type | table? ?? ?| type? ?| possible_keys? ???| key? ???| key_len | ref??| rows | Extra |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
|??1 | PRIMARY? ???|