MySql学习笔记（五）：explain-数据读取操作的操作类型

explain命令如下：

mysql> explain select * from t_blog;+----+-------------+--------+------+---------------+------+---------+------+------+-------+| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |+----+-------------+--------+------+---------------+------+---------+------+------+-------+|  1 | SIMPLE      | t_blog | ALL  | NULL          | NULL | NULL    | NULL |    7 |       |+----+-------------+--------+------+---------------+------+---------+------+------+-------+1 row in set

其中select_type列指明该条SQL的读取操作的操作类型。

select_type共有六种类型:simple、primmy、subQuery、derived、union、union result。

1、simple

表示该条sql是简单的select，不包含任何子查询和union，例：

mysql> explain select * from t_blog;+----+-------------+--------+------+---------------+------+---------+------+------+-------+| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |+----+-------------+--------+------+---------------+------+---------+------+------+-------+|  1 | SIMPLE      | t_blog | ALL  | NULL          | NULL | NULL    | NULL |    7 |       |+----+-------------+--------+------+---------------+------+---------+------+------+-------+1 row in set

2、primmy

查询中如果包含了任何一个子查询，最外层的查询就会被标记为primmy，例：

mysql> explain select * from t_blog where id = (select id from t_type where name = "JAVA");+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+| id | select_type | table  | type  | possible_keys | key     | key_len | ref   | rows | Extra       |+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+|  1 | PRIMARY     | t_blog | const | PRIMARY       | PRIMARY | 4       | const |    1 |             ||  2 | SUBQUERY    | t_type | ALL   | NULL          | NULL    | NULL    | NULL  |    4 | Using where |+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+2 rows in set

这条sql一共读取了两张表，t_type作为子查询被加载，t_blog作为最外部的读取操作，被标记为 PRIMMY。

3、subquery

查询中，在select或where自居中包含了子查询，该子查询就会被标记为subquery，如上例的t_type

4、derived

在from里列表中包含了子查询，该子查询会被标记为derived（衍生），例：

mysql> explain select * from t_blog inner join (select id from t_type) a on t_blog.typeId = a.id;+----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+| id | select_type | table      | type  | possible_keys | key     | key_len | ref  | rows | Extra       |+----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+|  1 | PRIMARY     |  | ALL   | NULL          | NULL    | NULL    | NULL |    4 |             ||  1 | PRIMARY     | t_blog     | ALL   | NULL          | NULL    | NULL    | NULL |    7 | Using where ||  2 | DERIVED     | t_type     | index | NULL          | PRIMARY | 4       | NULL |    4 | Using index |+----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+3 rows in set

t_type表读取时，出现在from语句中，因此被标记为derived。mysql会先将from语句的子查询的查询结果放到一张临时表中，然后再将最终的结果返回，所以，是很耗费内存的一种操作。值得注意的是，在第一行的table列出现了字样，表示该表是个衍生虚表，他的来源根据后面的数字"2"来寻找，这个"2"就是第一列的id，表示是id为2的衍生表的衍生虚表。

5、union

若sql中包含了union，第二个select会被标记为union

mysql> explain select * from t_blog b left join t_type t on b.typeId = t.id    union    select * from t_blog b1 right join t_type t1 on b1.typeId = t1.id;+------+--------------+------------+--------+---------------+---------+---------+---------------+------+-------+| id   | select_type  | table      | type   | possible_keys | key     | key_len | ref           | rows | Extra |+------+--------------+------------+--------+---------------+---------+---------+---------------+------+-------+|    1 | PRIMARY      | b          | ALL    | NULL          | NULL    | NULL    | NULL          |    7 |       ||    1 | PRIMARY      | t          | eq_ref | PRIMARY       | PRIMARY | 4       | blog.b.typeId |    1 |       ||    2 | UNION        | t1         | ALL    | NULL          | NULL    | NULL    | NULL          |    4 |       ||    2 | UNION        | b1         | ALL    | NULL          | NULL    | NULL    | NULL          |    7 |       || NULL | UNION RESULT |  | ALL    | NULL          | NULL    | NULL    | NULL          | NULL |       |+------+--------------+------------+--------+---------------+---------+---------+---------------+------+-------+5 rows in set

t1表和t2表都出现在union后的select，因此都被标及为union

6、union result

表示该查询是从union表中获取结果的select，是union的结果集，如上例，上条select就是获取的union的结果集