MySQL查询执行计划学习

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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)

7） 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)

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