Mysql-Explain（四）：輸出列-select_type

簡介

select_type	查詢的類型，主要用來區別普通查詢，聯合查詢，子查詢等複雜查詢	SIMPLE	簡單的select查詢，查詢中不包含子查詢或者UNION
		PRIMARY	查詢中若包含任何複雜的子部分，最外層的查詢則被標記爲PRIMARY
		SUBQUERY	在select或者where列表中包含了子查詢
		DERIVED	在from列表中包含的子查詢會被標記爲DERICED(衍生)，Mysql會遞歸地執行這些子查詢，然後把結果放到臨時表
		UNION	若第二個select語句出現在UNION之後，則被標記爲UNION。若UNION包含在from子句的子查詢中，外層select則被標記爲DERIVED
		UNION RESULT	從union表獲取結果的select

演示

• SIMPLE：簡單的select查詢，查詢中不包含子查詢或者UNION

  mysql> explain select * from student;
  +----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
  | id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra |
  +----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
  |  1 | SIMPLE      | student | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 1994898 |   100.00 | NULL  |
  +----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
  1 row in set, 1 warning (0.00 sec)
  
  mysql> explain select * from student
      -> left join school on student.school_id=school.id
      -> where school.id = 1;
  +----+-------------+---------+------------+-------+---------------+---------+---------+-------+---------+----------+-------------+
  | id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref   | rows    | filtered | Extra       |
  +----+-------------+---------+------------+-------+---------------+---------+---------+-------+---------+----------+-------------+
  |  1 | SIMPLE      | school  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |       1 |   100.00 | NULL        |
  |  1 | SIMPLE      | student | NULL       | ALL   | NULL          | NULL    | NULL    | NULL  | 1994898 |    10.00 | Using where |
  +----+-------------+---------+------------+-------+---------------+---------+---------+-------+---------+----------+-------------+
  2 rows in set, 1 warning (0.00 sec)
  

• PRIMARY：查詢中若包含任何複雜的子部分，最外層的查詢則被標記爲PRIMARY

  mysql> explain
      -> select * from student
      -> where student.school_id = (select id from school where id < 2);
  +----+-------------+---------+------------+-------+---------------+---------+---------+------+---------+----------+--------------------------+
  | 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，對錶school的查詢的select_type是subquery，表示這是個子查詢，對錶student的查詢的select_type是PRIMARY，表示這是主查詢。子查詢的id爲2，大於主查詢id，表示先執行了子查詢，再執行主查詢。

• SUBQUERY：在select或者where列表中包含了子查詢
  where列表的子查詢如上面PRIMARY例子所示，select列表的子查詢如下面所示：

  mysql> explain
      -> select student.*,(select name from school where id =1) from student
      -> where student.school_id = 1;
  +----+-------------+---------+------------+-------+---------------+---------+---------+-------+---------+----------+-------------+
  | 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       | const | PRIMARY       | PRIMARY | 4       | const |       1 |   100.00 | NULL        |
  +----+-------------+---------+------------+-------+---------------+---------+---------+-------+---------+----------+-------------+
  2 rows in set, 1 warning (0.03 sec)
  

• DERIVED：在from列表中包含的子查詢會被標記爲DERICED(衍生)，Mysql會遞歸地執行這些子查詢，然後把結果放到臨時表

  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)
  

  explain返回的第三行記錄中，select_type=DERIVED，表示對school的查詢結果會派生出新的臨時表，這張臨時表就是第一行記錄中顯示的<derived2>。

• UNION：若第二個select語句出現在UNION之後，則被標記爲UNION。若UNION包含在from子句的子查詢中，外層select則被標記爲DERIVED

  mysql> explain
      -> select student.*,school.name as school_name from student left join school on student.school_id=school.id where school.name is null
      -> union
      -> select student.*,major.name as major_name from student left join major on student.major_id=major.id where major.name is null;
  +----+--------------+------------+------------+--------+---------------+---------+---------+------------------------+---------+----------+-----------------+
  | 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 |   100.00 | NULL            |
  |  1 | PRIMARY      | school     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | mydb.student.school_id |       1 |    10.00 | Using where     |
  |  2 | UNION        | student    | NULL       | ALL    | NULL          | NULL    | NULL    | NULL                   | 1994898 |   100.00 | NULL            |
  |  2 | UNION        | major      | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | mydb.student.major_id  |       1 |    10.00 | Using where     |
  | NULL | UNION RESULT | <union1,2> | NULL       | ALL    | NULL          | NULL    | NULL    | NULL                   |    NULL |     NULL | Using temporary |
  +----+--------------+------------+------------+--------+---------------+---------+---------+------------------------+---------+----------+-----------------+
  5 rows in set, 1 warning (0.02 sec)
  

  從上面返回的結果中可以看出，對student和major的聯合查詢被標記爲了UNION，並且在Id上也被標記爲同一組。

• UNION RESULT：從union表獲取結果的select
  還是回到上面UNION的例子中去，第五行記錄所示，union上下的查詢語句會把結果合併到臨時表<union1,2>中，再從臨時表中讀取結果。