Mysql-Explain（三）：输出列-id

简介

在上一篇文章中，我们准备好了演示的表和数据，下面就来开始介绍第一个explain输出列：id.

字段	字段说明	字段值描述
id	select查询序列号，包含一组数字，表示查询中执行select子句或者操作表的顺序	id相同	执行顺序由上至下
		id不相同	如果是子查询，id的序号会递增，id的值越大被执行的优先级越高
		id相同和不相同都存在	如果id相同可以认为是一组，同一组id执行顺序由上至下，不同组之间，id值越大被执行的优先级越高

演示

• id相同：执行顺序由上至下

  mysql> explain select * from student,school,major
  -> where student.school_id = school.id and student.major_id = major.id
  -> and student.major_id = 321 limit 10;
  +----+-------------+---------+------------+--------+---------------+---------+---------+------------------------+---------+----------+-------------+
  | id | select_type | table   | partitions | type   | possible_keys | key     | key_len | ref                    | rows    | filtered | Extra       |
  +----+-------------+---------+------------+--------+---------------+---------+---------+------------------------+---------+----------+-------------+
  |  1 | SIMPLE      | major   | NULL       | const  | PRIMARY       | PRIMARY | 4       | const                  |       1 |   100.00 | NULL        |
  |  1 | SIMPLE      | student | NULL       | ALL    | NULL          | NULL    | NULL    | NULL                   | 1994898 |    10.00 | Using where |
  |  1 | SIMPLE      | school  | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | mydb.student.school_id |       1 |   100.00 | NULL        |
  +----+-------------+---------+------------+--------+---------------+---------+---------+------------------------+---------+----------+-------------+
  3 rows in set, 1 warning (0.00 sec)
  

  Id列的结果值全部是1，表示由上到下，先查询major表，刷选出major.id=321的记录，然后再查询student表，最后查询school表，这是MySQL优化器分析查询语句后得出的查询顺序。

• id不同：如果是子查询，id的序号会递增，id的值越大被执行的优先级越高

  mysql> explain
      -> select * from student
      -> where student.school_id = (select id from school where id < 2) limit 10;
  +----+-------------+---------+------------+-------+---------------+---------+---------+------+---------+----------+--------------------------+
  | id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref  | rows    | filtered | Extra                    |
  +----+-------------+---------+------------+-------+---------------+---------+---------+------+---------+----------+--------------------------+
  |  1 | PRIMARY     | student | NULL       | ALL   | NULL          | NULL    | NULL    | NULL | 1994898 |    10.00 | Using where              |
  |  2 | SUBQUERY    | school  | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |       1 |   100.00 | Using where; Using index |
  +----+-------------+---------+------------+-------+---------------+---------+---------+------+---------+----------+--------------------------+
  2 rows in set, 1 warning (0.00 sec)
  

  上图所示，id分别是1和2，2>1，表示子查询将优先被执行，之后再执行主查询，这个应该非常直观也非常容易理解。

• id同时存在相同和不相同：id相同为同一组，组内执行顺序由上至下，组间id值越大被执行的优先级越高

  mysql> explain
      -> select student.*,sc.name from
      -> (select * from school where school.id < 10 limit 2)sc,student
      -> where sc.id = student.school_id;
  +----+-------------+------------+------------+-------+---------------+---------+---------+------+---------+----------+----------------------------------------------------+
  | id | select_type | table      | partitions | type  | possible_keys | key     | key_len | ref  | rows    | filtered | Extra                                              |
  +----+-------------+------------+------------+-------+---------------+---------+---------+------+---------+----------+----------------------------------------------------+
  |  1 | PRIMARY     | <derived2> | NULL       | ALL   | NULL          | NULL    | NULL    | NULL |       2 |   100.00 | NULL                                               |
  |  1 | PRIMARY     | student    | NULL       | ALL   | NULL          | NULL    | NULL    | NULL | 1994898 |    10.00 | Using where; Using join buffer (Block Nested Loop) |
  |  2 | DERIVED     | school     | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |       9 |   100.00 | Using where                                        |
  +----+-------------+------------+------------+-------+---------------+---------+---------+------+---------+----------+----------------------------------------------------+
  3 rows in set, 1 warning (0.00 sec)
  

  上图所示id分别是1、1、2，表示查询被分成了两组，组2优先被查询，也就是对表school的子查询优先被执行了，然后再执行到组1。组1内的执行顺序由上至下，先执行对表<derived2>的查询，再执行对表student的查询。

  tips:

  • <derived2>是派生表2的意思，后面select_type会讲到。
  • form子查询语句中加LIMIT 2的原因是在MYSQL 5.7后，引入了derived_merge，一种查询优化技术，作用就是把派生表合并到外部的查询中，提高数据检索的效率。举例来说：“select * from (select * from a) aa where aa.id =1;”，在derived_merge下会被优化成"select * from a where a.id =1;"，但是当派生子查询存在以下操作时该特性无法生效：UNION 、GROUP BY、DISTINCT、LIMIT/OFFSET以及聚合操作。由于上面做实验的例子比较简单在derived_merge打开的情况下from子查询会合并到外层查询，无法演示效果，因此在子查询语句中加了limit，当然也可以通过设置属性"optimizer_switch='derived_merge=off"来关闭derived_merge。