MySQL EXPLAIN type 类型说明

时间:2021-02-23 14:29:19   收藏:0   阅读:0

数据库版本(select version();): 5.7.32-log

sakila 下载地址: https://dev.mysql.com/doc/index-other.html

type 表示 MySQL 在表中找到所需行的方式, 又称"访问类型".
ALL, index, range, ref, eq_ref, const, system, NULL (从左到右, 性能从差到好)

1. ALL

全表扫描(Full Table Scan), MySQL将遍历全表以找到匹配的行.

mysql> explain select * from film where rating =‘G‘;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | film  | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 1000 |    20.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

film 表中 rating 字段没有索引.

2. index

全索引扫描(Full Index Scan), index 与 ALL 区别为 index 类型只遍历索引树. MYSQL 遍历整个索引来查找匹配的行.

mysql> explain select title from film;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | film  | NULL       | index | NULL          | idx_title | 514     | NULL | 1000 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+

虽然 where 条件中没有用到索引, 但是要取出的列 title 是索引包含的列, 所以只要全扫描 title 索引即可, 直接使用索引树查找数据.

3. range

索引范围扫描, 常见于 ‘<‘, ‘<=‘, ‘>‘, ‘>=‘, ‘between‘ 等操作符.

mysql> explain select * from film where film_id > 100;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | film  | NULL       | range | PRIMARY       | PRIMARY | 2       | NULL |  900 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

因为 film_id 是索引, 所以只要查找索引的某个范围即可, 通过索引找到具体的数据.

4. ref

使用非唯一性索引或者唯一索引的前缀扫描, 返回匹配某个单独值的记录行.

mysql> explain select * from payment where customer_id = 10;
+----+-------------+---------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys      | key                | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | payment | NULL       | ref  | idx_fk_customer_id | idx_fk_customer_id | 2       | const |   25 |   100.00 | NULL  |
+----+-------------+---------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+

customer_id 在 payment 表中是非唯一性索引

5. eq_ref

类似ref, 区别就在使用的索引是唯一索引. 在联表查询中使用 primary key 或者 unique key 作为关联条件.

mysql> explain select * from film a left join film_text b on a.film_id = b.film_id;
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref              | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | ALL    | NULL          | NULL    | NULL    | NULL             | 1000 |   100.00 | NULL        |
|  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 2       | sakila.a.film_id |    1 |   100.00 | Using where |
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------------+

6. const/system

当 MySQL 对查询某部分进行优化, 并转换为一个常量时, 使用这些类型访问. 如将主键置于 where 列表中, MySQL 就能将该查询转换为一个常量, system 是 const 类型的特例, 当查询的表只有一行的情况下使用 system.

mysql> explain select * from film where film_id = 1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | film  | NULL       | const | PRIMARY       | PRIMARY | 2       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

7. NULL

MySQL 不用访问表或者索引就直接能到结果.

mysql> explain select 1 from dual where 1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+

dual是一个虚拟的表, 可以直接忽略.

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