一.语法
explain < table_name >
例如: explain select * from t3 where id=3952602;
二.explain输出解释
| +----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+ 1.id |
1.我的理解是SQL执行的顺利的标识,SQL从大到小的执行.
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 | | system | NULL | NULL | NULL | NULL | 1 | | | 2 | DERIVED | | system | NULL | NULL | NULL | NULL | 1 | | | 3 | DERIVED | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | | 1 | | +----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+ |
很显然这条SQL是从里向外的执行,就是从id=3 向上执行.
2. select_type就是select类型,可以有以下几种
(1) SIMPLE
简单SELECT(不使用UNION或子查询等) 例如:
| mysql> explain select * from t3 where id=3952602; +----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | const | 1 | | +----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+ |
(2). PRIMARY
我的理解是最外层的select.例如:
| mysql> explain select * from (select * from t3 where id=3952602) a ; +----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+ | 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | | | 2 | DERIVED | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | | 1 | | +----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+ |
