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) > 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;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
|> +----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 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) > explain select> *************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: film
type: index
possible_keys: NULL
key:> 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) > explain select * from payment where customer_id>300 and customer_id
key:> key_len: 2
ref: NULL
rows: 2637
Extra: Using where
1 row in set (0.00 sec)
(root@localhost) > 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:>
key:> 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) > explain select * from payment where customer_id=305\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: payment
type: ref
possible_keys:>
key:> 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> 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: 1
select_type: SIMPLE
table: customer
type: ref
possible_keys:>
key:> key_len: 137
ref: const
rows: 2
Extra: Using where; Using index
1 row in set (0.00 sec)
(root@localhost) >> 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: 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:>
key:> 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) > explain select * from film a join film_text b
-> on a.film_id=b.film_id;
+----+-------------+-------+--------+---------------+---------+---------+----------+------+-------+
|> +----+-------------+-------+--------+---------------+---------+---------+----------+------+-------+
|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> +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|> +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|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;
+----+-------------+------------+--------+---------------+-------+---------+-------+------+-------+
|> +----+-------------+------------+--------+---------------+-------+---------+-------+------+-------+
|1 | PRIMARY || 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) > explain select sysdate();
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|> +----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|1 | SIMPLE | NULL| NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)
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