MySql学习笔记（六）：扫描范围-蒲公英云

explain的type列表示该条查询的扫描范围，一共有七种，效果由上到下排列：

system>const>eq_ref>ref>range>index>all。

数据准备：

CREATE TABLE `t_blog` (
  `id` int(11) NOT NULL auto_increment,
  `title` varchar(50) default NULL,
  `typeId` int(11) default NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `title_index` (`title`),
  KEY `type_index` (`typeId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
CREATE TABLE `t_type` (
  `id` int(11) NOT NULL auto_increment,
  `name` varchar(20) default NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

1、system

该表中只有一行记录，这种情况在日常开发中很少见，不多赘述；

2、const

表示通过索引一次就找到了结果，用于扫描主键和唯一索引，例如：

mysql> explain select * from t_blog where id = 1;
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t_blog | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------+
1 row in set

在where子句中，id为主键且值为一个常数，在id索引中只有一条数据与之对应。

3、eq_ref

通过主键和唯一索引，只有一条数据与之匹配，例如：

mysql> explain select b.* from t_blog b left join t_type t on b.typeId = t.id;
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref           | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-------------+
|  1 | SIMPLE      | b     | ALL    | NULL          | NULL    | NULL    | NULL          |    7 |             |
|  1 | SIMPLE      | t     | eq_ref | PRIMARY       | PRIMARY | 4       | blog.b.typeId |    1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-------------+
2 rows in set

eq_ref和const都表示在唯一索引或主键的作用下，只找到一行与之匹配的数据。const表示按主键和唯一索引读取，eq_ref通常体现在连表上，按连表的主键和唯一索引读取。

4、ref

非唯一索引扫描，有多个行与之匹配

mysql> explain select * from t_blog where typeId = 4;
+----+-------------+--------+------+---------------+------------+---------+-------+------+-------------+
| id | select_type | table  | type | possible_keys | key        | key_len | ref   | rows | Extra       |
+----+-------------+--------+------+---------------+------------+---------+-------+------+-------------+
|  1 | SIMPLE      | t_blog | ref  | type_index    | type_index | 5       | const |    1 | Using where |
+----+-------------+--------+------+---------------+------------+---------+-------+------+-------------+
1 row in set

typeId是表的普通索引，即非唯一索引，与eq_ref最大的区别在于ref表示非唯一索引扫描。

5、range

表示范围，使用索引选择行，使用了 > < in beteen等

mysql> EXPLAIN select * from t_blog where id>2;
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t_blog | range | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using where |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
1 row in set

6、index

遍历索引树，读全表

mysql> EXPLAIN select id from t_blog;
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t_blog | index | NULL          | PRIMARY | 4       | NULL |    7 | Using index |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
1 row in set

只查询id，所以只遍历索引文件即可，不需要从硬盘中读取，比all快。

7、all

读全表，不使用任何索引，从硬盘中读数据，最慢

mysql> explain 
select * from t_blog;
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | t_blog | ALL  | NULL          | NULL | NULL    | NULL |    7 |       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
1 row in set