Mysql查看执行计划-explain(2)

+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+
 
(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     | <derived2> | system | NULL              | NULL    | NULL    | NULL |    1 |       |
|  2 | DERIVED     | t3         | const  | PRIMARY,idx_t3_id | PRIMARY | 4       |      |    1 |       |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
 
(3).UNION
 
UNION中的第二个或后面的SELECT语句.例如
mysql> explain select * from t3 where id=3952602 union all select * from t3 ;
+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+
| id | select_type  | table      | type  | possible_keys     | key     | key_len | ref   | rows | Extra |
+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+
|  1 | PRIMARY      | t3         | const | PRIMARY,idx_t3_id | PRIMARY | 4       | const |    1 |       |
|  2 | UNION        | t3         | ALL   | NULL              | NULL    | NULL    | NULL  | 1000 |       |
|NULL | UNION RESULT | <union1,2> | ALL   | NULL              | NULL    | NULL    | NULL  | NULL |       |
+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+
 
(4).DEPENDENT UNION
 
UNION中的第二个或后面的SELECT语句，取决于外面的查询
 
mysql> explain select * from t3 where id in (select id from t3 where id=3952602 union all select id from t3)  ;