MySQL explain type详解
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
indexa.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) > 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) > 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、indexMySQL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) > explain select title from film \G*************************** 1. row *************************** id: 1select_type: SIMPLE table: film type: indexpossible_keys: NULL key: idx_title key_len: 767 ref: NULL rows: 1000 Extra: Using index1 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) > explain select * from payment where customer_id>300 and customer_id<400\G*************************** 1. row *************************** id: 1select_type: SIMPLE table: payment type: rangepossible_keys: idx_fk_customer_id key: idx_fk_customer_id key_len: 2 ref: NULL rows: 2637 Extra: Using where1 row in set (0.00 sec)(root@localhost) > explain select * from payment where customer_id in (200,300,400)\G*************************** 1. row *************************** id: 1select_type: SIMPLE table: payment type: rangepossible_keys: idx_fk_customer_id key: idx_fk_customer_id key_len: 2 ref: NULL rows: 86 Extra: Using index condition1 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) > explain select * from payment where customer_id=305\G*************************** 1. row *************************** id: 1select_type: SIMPLE table: payment type: refpossible_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) > 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) > create index idx_fisrt_last_name on customer(first_name,last_name);Query OK, 599 rows affected (0.09 sec)Records: 599Duplicates: 0Warnings: 0(root@localhost) > 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) > explain select first_name from customer where first_name='JESSIE'\G*************************** 1. row *************************** id: 1select_type: SIMPLE table: customer type: refpossible_keys: idx_fisrt_last_name key: idx_fisrt_last_name key_len: 137 ref: const rows: 2 Extra: Using where; Using index1 row in set (0.00 sec)(root@localhost) > alter table customer drop index idx_fisrt_last_name;Query OK, 599 rows affected (0.03 sec)Records: 599Duplicates: 0Warnings: 0--下面演示出现在join是ref的示例(root@localhost) > explain select b.*,a.* from payment a inner join -> customer b on a.customer_id=b.customer_id\G*************************** 1. row *************************** id: 1select_type: SIMPLE table: b type: ALLpossible_keys: PRIMARY key: NULL key_len: NULL ref: NULL rows: 599 Extra: NULL*************************** 2. row *************************** id: 1select_type: SIMPLE table: a type: refpossible_keys: idx_fk_customer_id key: idx_fk_customer_id key_len: 2 ref: sakila.b.customer_id rows: 13 Extra: NULL2 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) > 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) > 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) > create table t1(id int,ename varchar(20) unique);Query OK, 0 rows affected (0.05 sec)(root@localhost) > insert into t1 values(1,'robin'),(2,'jack'),(3,'henry');Query OK, 3 rows affected (0.00 sec)Records: 3Duplicates: 0Warnings: 0(root@localhost) > 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=NULLMySQL不用访问表或者索引就可以直接得到结果(root@localhost) > 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)
页:
[1]