Mysql-Explain（七）：輸出列-ref、rows

簡介

ref	哪些列或者常量被用做索引列上的值
rows	根據表的統計信息和索引的使用情況，大致估算查詢結果所需要讀取記錄的行數

演示

• ref：哪些列或者常量被用做索引列上的值

  mysql> explain select * from student where school_id = 1;
  +----+-------------+---------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+
  | id | select_type | table   | partitions | type | possible_keys       | key                 | key_len | ref   | rows | filtered | Extra |
  +----+-------------+---------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+
  |  1 | SIMPLE      | student | NULL       | ref  | ik_schoolId_majorId | ik_schoolId_majorId | 5       | const | 1960 |   100.00 | NULL  |
  +----+-------------+---------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+
  1 row in set, 1 warning (0.07 sec)
  

  school_id =1，1是常量，ref=const

  mysql> explain select * from student left join school on student.school_id = school.id;
  +----+-------------+---------+------------+--------+---------------+---------+---------+------------------------+---------+----------+-------+
  | id | select_type | table   | partitions | type   | possible_keys | key     | key_len | ref                    | rows    | filtered | Extra |
  +----+-------------+---------+------------+--------+---------------+---------+---------+------------------------+---------+----------+-------+
  |  1 | SIMPLE      | student | NULL       | ALL    | NULL          | NULL    | NULL    | NULL                   | 1994142 |   100.00 | NULL  |
  |  1 | SIMPLE      | school  | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | mydb.student.school_id |       1 |   100.00 | NULL  |
  +----+-------------+---------+------------+--------+---------------+---------+---------+------------------------+---------+----------+-------+
  2 rows in set, 1 warning (0.00 sec)
  

  上面例子ref的值是mydb.student.school_id，因爲表school被關聯查詢的時候，使用了主鍵索引，而值則使用了驅動表的mydb.student.school_id列。

• rows：根據表的統計信息和索引的使用情況，大致估算查詢結果所需要讀取記錄的行數；filtered：表示示存儲引擎返回的數據在server層過濾後，剩下多少滿足查詢的記錄數量的比例

  mysql> explain select * from student where school_id < 4 and major_id = 3;
  +----+-------------+---------+------------+------+--------------------------------+------------+---------+-------+------+----------+-------------+
  | id | select_type | table   | partitions | type | possible_keys                  | key        | key_len | ref   | rows | filtered | Extra       |
  +----+-------------+---------+------------+------+--------------------------------+------------+---------+-------+------+----------+-------------+
  |  1 | SIMPLE      | student | NULL       | ref  | ik_schoolId_majorId,ik_majorId | ik_majorId | 5       | const | 4076 |     0.77 | Using where |
  +----+-------------+---------+------------+------+--------------------------------+------------+---------+-------+------+----------+-------------+
  1 row in set, 1 warning (0.07 sec)
  

  上面explain的分析結果顯示，在存儲引擎層面上大概需要掃描4076行記錄，使用的索引是ik_majorId，所以數據在server層還需要對shcool_id條件進行過濾，過濾後的匹配比例是0.77。在我們平時的sql優化中rows一般是越小越好，代表所需掃描的表記錄少，而filtered是越大越好，表明掃描的無用數據少，沒有過多的多餘的IO消耗。