MySQL查询执行计划学习

SELECT_TYPE：

1）  SIMPLE：简单的SELECT，不实用UNION或者子查询

mysql> explain select * from t2;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | t2  | ALL   | NULL | NULL          | NULL | NULL    | 100  | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

2）  PRIMARY：最外层SELECT。

mysql> explain select * from (select * from t2 where id2=2) b;
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
| 1  | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL | 100  | NULL        |
| 2  | DERIVED     | t2         | ALL  | NULL          | NULL | NULL    | NULL | 100  | Using where |
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
2 rows in set (0.00 sec)

3）  UNION：第二层，在SELECT之后使用了UNION。

mysql> explain select * from t1 union select * from t2;
+----+--------------+------------+------+---------------+------+---------+------+-------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+------+---------------+------+---------+------+-------+-----------------+
| 1 | PRIMARY | t1 | ALL | NULL | NULL | NULL | NULL | 10208 | NULL |
| 2 | UNION | t2 | ALL | NULL | NULL | NULL | NULL | 100 | NULL |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------+---------------+------+---------+------+-------+-----------------+
3 rows in set (0.00 sec)

4）  DEPENDENT UNION：UNION语句中的第二个SELECT，依赖于外部子查询。

mysql> explain select * from t1 where id1 in (select id2 from t2 where id2<10 union select id3 from t3 where id3<5);
+----+--------------------+------------+------+---------------+------+---------+------+-------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------+------+---------------+------+---------+------+-------+--------------------------+
| 1 | PRIMARY | t1 | ALL | NULL | NULL | NULL | NULL | 10208 | Using where |
| 2 | DEPENDENT SUBQUERY | t2 | ALL | NULL | NULL | NULL | NULL | 100 | Using where |
| 3 | DEPENDENT UNION | t3 | ref | id3 | id3 | 5 | func | 1 | Using where; Using index |
| NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+------------+------+---------------+------+---------+------+-------+--------------------------+
4 rows in set (0.00 sec)

5）  UNION RESULT：UNION的结果。

mysql> explain select * from t1 union select * from t2;
+----+--------------+------------+------+---------------+------+---------+------+-------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+------+---------------+------+---------+------+-------+-----------------+
| 1 | PRIMARY | t1 | ALL | NULL | NULL | NULL | NULL | 10208 | NULL |
| 2 | UNION | t2 | ALL | NULL | NULL | NULL | NULL | 100 | NULL |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------+---------------+------+---------+------+-------+-----------------+
3 rows in set (0.00 sec)

6）  SUBQUERY：子查询中的第一个SELECT。

mysql> explain select * from t1 where id1=(select id2 from t2 where id2=2);
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | PRIMARY | t1 | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
| 2 | SUBQUERY | t2 | ALL | NULL | NULL | NULL | NULL | 100 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
2 rows in set (0.00 sec)

8）  DERIVED：被驱动的SELECT子查询

mysql> explain select * from (select * from t2 where id2=2) b;
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
| 1  | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL | 100  | NULL        |
| 2  | DERIVED     | t2         | ALL  | NULL          | NULL | NULL    | NULL | 100  | Using where |
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
2 rows in set (0.00 sec)