mysql union ,UNION RESULT

mysql> explain select * from t100 union all select * from t200;
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra           |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
|  1 | PRIMARY      | t100       | ALL  | NULL          | NULL | NULL    | NULL |    5 | NULL            |
|  2 | UNION        | t200       | ALL  | NULL          | NULL | NULL    | NULL |    5 | NULL            |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL | Using temporary |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
3 rows in set (0.00 sec)

SELECT类型:

id	SELECT识别符。这是SELECT的查询序列号
select_type
SELECT类型,可以为以下任何一种:
SIMPLE:简单SELECT(不使用UNION或子查询)
PRIMARY:最外面的SELECT
UNION:UNION中的第二个或后面的SELECT语句
DEPENDENT UNION:UNION中的第二个或后面的SELECT语句,取决于外面的查询
UNION RESULT:UNION 的结果
SUBQUERY:子查询中的第一个SELECT
DEPENDENT SUBQUERY:子查询中的第一个SELECT,取决于外面的查询
DERIVED:导出表的SELECT(FROM子句的子查询)