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NULLindex_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

(root@localhost) [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

(root@localhost) [sakila]> explain select count(description) from film;

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

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

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

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

(root@localhost) [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

(root@localhost) [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)

(root@localhost) [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

(root@localhost) [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

(root@localhost) [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

(root@localhost) [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

(root@localhost) [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)

(root@localhost) [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)

(root@localhost) [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

(root@localhost) [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。

(root@localhost) [sakila]> explain select * from film a join film_text b

-> on a.film_id=b.film_id;

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

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

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

(root@localhost) [sakila]> explain select title from film where film_id=5;

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

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

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

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

(root@localhost) [sakila]> create table t1(id int,ename varchar(20) unique);

Query OK, 0 rows affected (0.05 sec)

(root@localhost) [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

(root@localhost) [sakila]> explain select * from (select * from t1 where ename='robin')x;

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

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

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

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

(root@localhost) [sakila]> explain select sysdate();

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

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

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

1 row in set (0.00 sec)

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