| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | t1 | const | PRIMARY,idx_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 t1 where id=888 union all select * from t1 ;
+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+
| 1 | PRIMARY | t1 | const | PRIMARY,idx_id | PRIMARY | 4 | const | 1 | |
| 2 | UNION | t1 | 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 t1 where id in (select id from t1 where id=888 union all select id from t1) ;
+----+--------------------+------------+--------+-------------------+---------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------+--------+-------------------+---------+---------+-------+------+--------------------------+
| 1 | PRIMARY | t1 | ALL | NULL | NULL | NULL | NULL | 1000 | Using where |
| 2 | DEPENDENT SUBQUERY | t1 | const | PRIMARY,idx_id | PRIMARY | 4 | const | 1 | Using index |
| 3 | DEPENDENT UNION | t1 | eq_ref | PRIMARY,idx_id | PRIMARY | 4 | func | 1 | Using where; Using index |
|NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------------+------------+--------+-------------------+---------+---------+-------+------+--------------------------+
(4).UNION RESULT
UNION的结果。
mysql> explain select * from t1 where id=888 union all select * from t1 ;
+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+
| 1 | PRIMARY | t1 | const | PRIMARY,idx_id | PRIMARY | 4 | const | 1 | |