Mysql-Explain（三）：輸出列-id

簡介

在上一篇文章中，我們準備好了演示的表和數據，下面就來開始介紹第一個explain輸出列：id.

字段	字段說明	字段值描述
id	select查詢序列號，包含一組數字，表示查詢中執行select子句或者操作表的順序	id相同	執行順序由上至下
		id不相同	如果是子查詢，id的序號會遞增，id的值越大被執行的優先級越高
		id相同和不相同都存在	如果id相同可以認爲是一組，同一組id執行順序由上至下，不同組之間，id值越大被執行的優先級越高

演示

• id相同：執行順序由上至下

  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列的結果值全部是1，表示由上到下，先查詢major表，刷選出major.id=321的記錄，然後再查詢student表，最後查詢school表，這是MySQL優化器分析查詢語句後得出的查詢順序。

• id不同：如果是子查詢，id的序號會遞增，id的值越大被執行的優先級越高

  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)
  

  上圖所示，id分別是1和2，2>1，表示子查詢將優先被執行，之後再執行主查詢，這個應該非常直觀也非常容易理解。

• id同時存在相同和不相同：id相同爲同一組，組內執行順序由上至下，組間id值越大被執行的優先級越高

  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)
  

  上圖所示id分別是1、1、2，表示查詢被分成了兩組，組2優先被查詢，也就是對錶school的子查詢優先被執行了，然後再執行到組1。組1內的執行順序由上至下，先執行對錶<derived2>的查詢，再執行對錶student的查詢。

  tips:

  • <derived2>是派生表2的意思，後面select_type會講到。
  • form子查詢語句中加LIMIT 2的原因是在MYSQL 5.7後，引入了derived_merge，一種查詢優化技術，作用就是把派生表合併到外部的查詢中，提高數據檢索的效率。舉例來說：“select * from (select * from a) aa where aa.id =1;”，在derived_merge下會被優化成"select * from a where a.id =1;"，但是當派生子查詢存在以下操作時該特性無法生效：UNION 、GROUP BY、DISTINCT、LIMIT/OFFSET以及聚合操作。由於上面做實驗的例子比較簡單在derived_merge打開的情況下from子查詢會合併到外層查詢，無法演示效果，因此在子查詢語句中加了limit，當然也可以通過設置屬性"optimizer_switch='derived_merge=off"來關閉derived_merge。