Mysql-Explain（七）：输出列-ref、rows

简介

ref	哪些列或者常量被用做索引列上的值
rows	根据表的统计信息和索引的使用情况，大致估算查询结果所需要读取记录的行数

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，因为表school被关联查询的时候，使用了主键索引，而值则使用了驱动表的mydb.student.school_id列。

• rows：根据表的统计信息和索引的使用情况，大致估算查询结果所需要读取记录的行数；filtered：表示示存储引擎返回的数据在server层过滤后，剩下多少满足查询的记录数量的比例

  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的分析结果显示，在存储引擎层面上大概需要扫描4076行记录，使用的索引是ik_majorId，所以数据在server层还需要对shcool_id条件进行过滤，过滤后的匹配比例是0.77。在我们平时的sql优化中rows一般是越小越好，代表所需扫描的表记录少，而filtered是越大越好，表明扫描的无用数据少，没有过多的多余的IO消耗。