MySQL explain type详解

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  对于mysql执行计划的获取,我们可以通过explain方式来查看,explain方式看似简单,实际上包含的内容很多,尤其是输出结果中的type类型列。理解这些不同的类型,对于我们SQL优化举足轻重。

一、EXPLAIN 语句中type列的值

类型含义
system表只有一行
const表最多只有一行匹配,通用用于主键或者唯一索引比较时
eq_ref

每次与之前的表合并行都只在该表读取一行,这是除了system,const之外最好的一种,特点是使用=,而且索引的所有部分都参与join且索引是主键或非空唯一键的索引

ref如果每次只匹配少数行,那就是比较好的一种,使用=或<=>,可以是左覆盖索引或非主键或非唯一键
fulltext全文搜索
ref_or_null与ref类似,但包括NULL
index_merge

表示出现了索引合并优化(包括交集,并集以及交集之间的并集),但不包括跨表和全文索引。

这个比较复杂,目前的理解是合并单表的范围索引扫描(如果成本估算比普通的range要更优的话

unique_subquery

在in子查询中,就是value in (select...)把形如“select unique_key_column”的子查询替换。

PS:所以不一定in子句中使用子查询就是低效的!

index_subquery同上,但把形如”select non_unique_key_column“的子查询替换
range 常数值的范围
index

a.当查询是索引覆盖的,即所有数据均可从索引树获取的时候(Extra中有Using Index);

b.以索引顺序从索引中查找数据行的全表扫描(无 Using Index);

c.如果Extra中Using Index与Using Where同时出现的话,则是利用索引查找键值的意思;

d.如单独出现,则是用读索引来代替读行,但不用于查找

all全表扫描


二、连接类型部分示例

1、all

-- 环境描述

([email protected]) [sakila]> show variables like ‘version‘;

+---------------+--------+

| Variable_name | Value  |

+---------------+--------+

| version     | 5.6.26 |

+---------------+--------+

MySQL采取全表遍历的方式来返回数据行,等同于Oracle的full table scan

([email protected]) [sakila]> explain select count(description) from film;

+----+-------------+-------+------+---------------+------+---------+------+------+-------+

| id | select_type | table | type | possible_keys | key  | key_len | ref | rows | Extra |

+----+-------------+-------+------+---------------+------+---------+------+------+-------+

| 1 | SIMPLE    | film  | ALL  | NULL      | NULL | NULL   | NULL | 1000 | NULL  |

+----+-------------+-------+------+---------------+------+---------+------+------+-------+

2、index

MySQL采取索引全扫描的方式来返回数据行,等同于Oracle的full index scan

([email protected]) [sakila]> explain select title from film \G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: film

         type: index

possible_keys: NULL

          key: idx_title

      key_len: 767

          ref: NULL

         rows: 1000

        Extra: Using index

1 row in set (0.00 sec)

3、  range

索引范围扫描,对索引的扫描开始于某一点,返回匹配值域的行,常见于between、<、>等的查询

等同于Oracle的index range scan

([email protected]) [sakila]> explain select * from payment where customer_id>300 and customer_id<400\G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: payment

         type: range

possible_keys: idx_fk_customer_id

          key: idx_fk_customer_id

      key_len: 2

          ref: NULL

         rows: 2637

        Extra: Using where

1 row in set (0.00 sec)

([email protected]) [sakila]> explain select * from payment where customer_id in (200,300,400)\G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: payment

         type: range

possible_keys: idx_fk_customer_id

          key: idx_fk_customer_id

      key_len: 2

          ref: NULL

         rows: 86

        Extra: Using index condition

1 row in set (0.00 sec)

4、ref

非唯一性索引扫描或者,返回匹配某个单独值的所有行。常见于使用非唯一索引即唯一索引的非唯一前缀进行的查找

([email protected]) [sakila]> explain select * from payment where customer_id=305\G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: payment

         type: ref

possible_keys: idx_fk_customer_id

          key: idx_fk_customer_id

      key_len: 2

          ref: const

         rows: 25

        Extra: 

1 row in set (0.00 sec)

idx_fk_customer_id为表payment上的外键索引,且存在多个不不唯一的值,如下查询

([email protected]) [sakila]> select customer_id,count(*) from payment group by customer_id

    -> limit 2;

+-------------+----------+

| customer_id | count(*) |

+-------------+----------+

|       1 |     32 |

|       2 |     27 |

+-------------+----------+

-- 下面是非唯一前缀索引使用ref的示例

([email protected]) [sakila]> create index idx_fisrt_last_name on customer(first_name,last_name);

Query OK, 599 rows affected (0.09 sec)

Records: 599  Duplicates: 0  Warnings: 0

([email protected]) [sakila]> select first_name,count(*) from customer group by first_name 

    -> having count(*)>1 limit 2;

+------------+----------+

| first_name | count(*) |

+------------+----------+

| JAMIE    |      2 |

| JESSIE  |      2 |

+------------+----------+

2 rows in set (0.00 sec)

([email protected]) [sakila]> explain select first_name from customer where first_name=‘JESSIE‘\G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: customer

         type: ref

possible_keys: idx_fisrt_last_name

          key: idx_fisrt_last_name

      key_len: 137

          ref: const

         rows: 2

        Extra: Using where; Using index

1 row in set (0.00 sec)

([email protected]) [sakila]> alter table customer drop index idx_fisrt_last_name;

Query OK, 599 rows affected (0.03 sec)

Records: 599  Duplicates: 0  Warnings: 0

--下面演示出现在join是ref的示例

([email protected]) [sakila]> explain select b.*,a.* from payment a inner join

    -> customer b on a.customer_id=b.customer_id\G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: b

         type: ALL

possible_keys: PRIMARY

          key: NULL

      key_len: NULL

          ref: NULL

         rows: 599

        Extra: NULL

*************************** 2. row ***************************

           id: 1

  select_type: SIMPLE

        table: a

         type: ref

possible_keys: idx_fk_customer_id

          key: idx_fk_customer_id

      key_len: 2

          ref: sakila.b.customer_id

         rows: 13

        Extra: NULL

2 rows in set (0.01 sec)

5、eq_ref

类似于ref,其差别在于使用的索引为唯一索引,对于每个索引键值,表中只有一条记录与之匹配。

多见于主键扫描或者索引唯一扫描。

([email protected]) [sakila]> explain select * from film a join film_text b 

    -> on a.film_id=b.film_id;

+----+-------------+-------+--------+---------------+---------+---------+----------+------+-------+

| id | select_type | table | type   | possible_keys | key   | key_len | ref    | rows | Extra     |

+----+-------------+-------+--------+---------------+---------+---------+----------+------+-------+

|  1 | SIMPLE   | b     | ALL   | PRIMARY    | NULL  | NULL  | NULL   | 1000 | NULL  |

|  1 | SIMPLE   | a     | eq_ref | PRIMARY    | PRIMARY | 2   | sakila.b.film_id | 1 | Using where |

+----+-------------+-------+--------+---------------+---------+---------+-----------+------+------+

([email protected]) [sakila]> explain select title from film where film_id=5;

+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+

| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |

+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+

|  1 | SIMPLE      | film  | const | PRIMARY  | PRIMARY | 2   | const |    1 | NULL  |

+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+

6、const、system:

当MySQL对查询某部分进行优化,这个匹配的行的其他列值可以转换为一个常量来处理。

如将主键或者唯一索引置于where列表中,MySQL就能将该查询转换为一个常量

([email protected]) [sakila]> create table t1(id int,ename varchar(20) unique);

Query OK, 0 rows affected (0.05 sec)

([email protected]) [sakila]> insert into t1 values(1,‘robin‘),(2,‘jack‘),(3,‘henry‘);

Query OK, 3 rows affected (0.00 sec)

Records: 3  Duplicates: 0  Warnings: 0

([email protected]) [sakila]> explain select * from (select * from t1 where ename=‘robin‘)x;

+----+-------------+------------+--------+---------------+-------+---------+-------+------+-------+

| id | select_type | table      | type   | possible_keys | key   | key_len | ref   | rows | Extra |

+----+-------------+------------+--------+---------------+-------+---------+-------+------+-------+

|  1 | PRIMARY | <derived2> | system | NULL | NULL  | NULL    | NULL  |    1 | NULL  |

|  2 | DERIVED  | t1     | const  | ename   | ename | 23      | const |    1 | NULL  |

+----+-------------+------------+--------+---------------+-------+---------+-------+------+-------+

2 rows in set (0.00 sec)

7、type=NULL

MySQL不用访问表或者索引就可以直接得到结果

([email protected]) [sakila]> explain select sysdate();

+----+-------------+-------+------+---------------+------+---------+------+------+----------------+

| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |

+----+-------------+-------+------+---------------+------+---------+------+------+----------------+

|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | No tables used |

+----+-------------+-------+------+---------------+------+---------+------+------+----------------+

1 row in set (0.00 sec)

本文出自 “Jeff on the way” 博客,请务必保留此出处http://nrain.blog.51cto.com/11930278/1881208

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