SQL优化连弹三之explain

MySQL提供了EXPLAIN或DESC命令,获取MYSQL如何执行SELECT语句得信息,包括在SELECT语句执行过程中表如何连接和连接得顺序。

为了方便演示，大家可以下载官方sakila示例数据库文件

mysql> explain select * from film;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 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 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

• select_type表示select的类型,常见的取值有SIMPLE(简单表，即不使用表连接或者子查询)、PRIMARY(主查询，即外层的查询)、UNION、SUBQUERY(子查询的第一个SELECT)
• table表示对应的表
• partitions 表的分区
• possible_keys 此次查询可能用到的索引
• key 此次查询确切用的索引
• key_len 使用到索引字段得长度
• rows扫描行的数量
• filtered此次查询条件所过滤数据的百分比
• extra执行情况的说明和描述
• type表示MySQL在表中找到所需行的方式，或者叫访问类型。

接下来我们具体根据type进行讲解。

TYPE

从左到右，性能由差变好。

ALL全表扫描

type=All,全表扫描，MySQL遍历全表来找到匹配行

mysql> explain select * from film;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 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 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

Index索引全扫描

type=index,索引全扫描，MySQL遍历整个索引来查询匹配的行。

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 | 767     | NULL | 1000 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

range索引范围扫描

type=range.索引范围扫描,常见与<、<=、>、>=、between等操作符

(1)唯一索引和非唯一索引查询带双操作符

mysql> EXPLAIN select * from payment where customer_id >=300 and customer_id <= 350;
+----+-------------+---------+------------+-------+--------------------+--------------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys      | key                | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+--------------------+--------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | payment | NULL       | range | idx_fk_customer_id | idx_fk_customer_id | 2       | NULL | 1350 |   100.00 | Using index condition |
+----+-------------+---------+------------+-------+--------------------+--------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

(2)唯一索引带单操作符

mysql> EXPLAIN select * from payment where payment_id > 10;
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | payment | NULL       | range | PRIMARY       | PRIMARY | 2       | NULL | 8043 |   100.00 | Using where |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

(3)字符串索引也支持索引范围扫描，但只支持右%,不支持左%或左右%。

mysql> EXPLAIN select * from film where title like "ACADEMY%";
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | film  | NULL       | range | idx_title     | idx_title | 767     | NULL |    1 |   100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

index_merge

index_merge使用了索引合并得优化方法。customer_id为主键，store_id为普通索引。

mysql> EXPLAIN select * from customer where customer_id=1 or store_id=1;
+----+-------------+----------+------------+-------------+-------------------------+-------------------------+---------+------+------+----------+---------------------------------------------------+
| id | select_type | table    | partitions | type        | possible_keys           | key                     | key_len | ref  | rows | filtered | Extra                                             |
+----+-------------+----------+------------+-------------+-------------------------+-------------------------+---------+------+------+----------+---------------------------------------------------+
|  1 | SIMPLE      | customer | NULL       | index_merge | PRIMARY,idx_fk_store_id | PRIMARY,idx_fk_store_id | 2,1     | NULL |  330 |   100.00 | Using union(PRIMARY,idx_fk_store_id); Using where |
+----+-------------+----------+------------+-------------+-------------------------+-------------------------+---------+------+------+----------+---------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

ref非唯一索引扫描

(1)
type=ref,使用非唯一索引扫描,返回匹配某个单独值得记录行。

mysql> EXPLAIN select * from payment where customer_id=1;
+----+-------------+---------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
| 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 |   32 |   100.00 | NULL  |
+----+-------------+---------+------------+------+--------------------+--------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

注1：如果匹配值得记录行为所有记录，type会记为All
注2:主键索引为特殊的唯一索引，所以这里用主键去匹配值得时候执行计划返回的type不是ref。

(2)type=ref也常常出现在join操作中：

mysql> EXPLAIN select * from customer INNER JOIN payment on customer.customer_id=payment.customer_id;
+----+-------------+----------+------------+------+--------------------+--------------------+---------+-----------------------------+------+----------+-------+
| id | select_type | table    | partitions | type | possible_keys      | key                | key_len | ref                         | rows | filtered | Extra |
+----+-------------+----------+------------+------+--------------------+--------------------+---------+-----------------------------+------+----------+-------+
|  1 | SIMPLE      | customer | NULL       | ALL  | PRIMARY            | NULL               | NULL    | NULL                        |  599 |   100.00 | NULL  |
|  1 | SIMPLE      | payment  | NULL       | ref  | idx_fk_customer_id | idx_fk_customer_id | 2       | sakila.customer.customer_id |   26 |   100.00 | NULL  |
+----+-------------+----------+------------+------+--------------------+--------------------+---------+-----------------------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)

可以看到在customer中是全表扫描，在表payment中customer_id作为非唯一索引扫描。

eq_ref

type=eq_ref,类似于ref,区别在使用索引是唯一索引，简单来说，就是多表连接中使用primary key或者unique index作为关联条件。
主键相连得例子：

mysql> EXPLAIN select * from film INNER JOIN film_text on film.film_id=film_text.film_id;
+----+-------------+-----------+------------+--------+---------------+---------+---------+---------------------+------+----------+-------------+
| id | select_type | table     | partitions | type   | possible_keys | key     | key_len | ref                 | rows | filtered | Extra       |
+----+-------------+-----------+------------+--------+---------------+---------+---------+---------------------+------+----------+-------------+
|  1 | SIMPLE      | film      | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL                | 1000 |   100.00 | NULL        |
|  1 | SIMPLE      | film_text | NULL       | eq_ref | PRIMARY       | PRIMARY | 2       | sakila.film.film_id |    1 |   100.00 | Using where |
+----+-------------+-----------+------------+--------+---------------+---------+---------+---------------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

const/system

type=const/system,单表中最多一个匹配行，查询起来非常迅速。const是根据primary key或者唯一索引unique index进行查询,system表示系统只有一条数据，这是一个特殊得const类型。

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  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

NULL

type=null,MySQL不用访问表或者索引，直接就能够得到结果。

mysql> EXPLAIN select 1 from dual;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

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