mysql explain type连接类型示例

偏执的太偏执、 2022-08-09 13:56 153阅读 0赞

> 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，本文仅描述explian输出结果中的type列，同时给出其演示。
> 
> 有关explian输出的全描述，可以参考：[MySQL EXPLAIN SQL 输出信息描述][MySQL EXPLAIN SQL]

## 一、EXPLAIN 语句中type列的值 ##

type：
        连接类型
        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
    (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;
    +----+-------------+-------+------+---------------+------+---------+------+------+-------+
    | 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
    (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; +----+-------------+-------+--------+---------------+---------+---------+------------------+------+-------------+
    | 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 | +----+-------------+-------+--------+---------------+---------+---------+------------------+------+-------------+
    
    
    (root@localhost) [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
    (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;
    +----+-------------+------------+--------+---------------+-------+---------+-------+------+-------+
    | 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
    (root@localhost) [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)

[MySQL EXPLAIN SQL]: http://blog.csdn.net/leshami/article/details/49069199