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02-explain详解与索引最佳实践

索引详解实践最佳 02 explain

2023-09-11 14:18:28 时间

mysql安装文档参考：https://blog.csdn.net/yougoule/article/details/56680952

一、explain的使用和详解

1.1 explain工具介绍

使用EXPLAIN关键字可以模拟优化器执行SQL语句，分析你的查询语句或是结构的性能瓶颈在 select 语句之前增加 explain 关键字，MySQL 会在查询上设置一个标记，执行查询会返回执行计划的信息，而不是执行这条SQL
注意：如果 from 中包含子查询，仍会执行该子查询，将结果放入临时表中

1.2 Explain分析示例

参考官方文档：https://dev.mysql.com/doc/refman/5.7/en/explain-output.html
示例表：

DROP TABLE IF EXISTS `actor`;

CREATE TABLE `actor` (
  `id` int(11) NOT NULL,
  `name` varchar(45) DEFAULT NULL,
  `update_time` datetime DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

/*Data for the table `actor` */

insert  into `actor`(`id`,`name`,`update_time`) values 

(1,'a','2021-10-05 10:09:05'),

(2,'b','2021-10-22 10:09:13'),

(3,'c','2021-10-24 10:09:20');

/*Table structure for table `film` */

DROP TABLE IF EXISTS `film`;

CREATE TABLE `film` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

/*Data for the table `film` */

insert  into `film`(`id`,`name`) values 

(3,'film0'),

(1,'film1'),

(2,'film2');

/*Table structure for table `film_actor` */

DROP TABLE IF EXISTS `film_actor`;

CREATE TABLE `film_actor` (
  `id` int(11) NOT NULL,
  `film_id` int(11) NOT NULL,
  `actor_id` int(11) NOT NULL,
  `remark` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_film_actor_id` (`film_id`,`actor_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

/*Data for the table `film_actor` */

insert  into `film_actor`(`id`,`film_id`,`actor_id`,`remark`) values 

(1,1,1,NULL),

(2,1,2,NULL),

(3,2,1,NULL);

小试牛刀：

mysql> explain select * from actor;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | actor | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

在查询中的每个表会输出一行，如果有两个表通过 join 连接查询，那么会输出两行

1.3 explain 两个变种

1）explain extended

会在 explain 的基础上额外提供一些查询优化的信息。紧随其后通过 show warnings 命令可以得到优化后的查询语句，从而看出优化器优化了什么。额外还有 filtered 列，是一个百分比的值，
rows * filtered/100 可以估算出将要和 explain 中前一个表进行连接的行数（前一个表指 explain 中的id值比当前表id值小的表）。

mysql> explain extended select * from film where id = 1;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | film  | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 |       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> show warnings;
+-------+------+-----------------------------------------------------------------+
| Level | Code | Message                                                         |
+-------+------+-----------------------------------------------------------------+
| Note  | 1003 | select '1' AS `id`,'film1' AS `name` from `test`.`film` where 1 |
+-------+------+-----------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>  select '1' AS `id`,'film1' AS `name` from `test`.`film` where 1;
+----+-------+
| id | name  |
+----+-------+
| 1  | film1 |
| 1  | film1 |
| 1  | film1 |
+----+-------+
3 rows in set (0.00 sec)

演示filtered列的计算公式：

mysql> select * from actor;
+----+------+---------------------+
| id | name | update_time         |
+----+------+---------------------+
|  1 | a    | 2021-10-05 10:09:05 |
|  2 | b    | 2021-10-22 10:09:13 |
|  3 | c    | 2021-10-24 10:09:20 |
|  4 | d    | 2021-10-24 12:51:52 |
+----+------+---------------------+
4 rows in set (0.00 sec)

mysql> select * from film_actor;
+----+---------+----------+--------+
| id | film_id | actor_id | remark |
+----+---------+----------+--------+
|  1 |       1 |        1 | NULL   |
|  2 |       1 |        2 | NULL   |
|  3 |       2 |        1 | NULL   |
+----+---------+----------+--------+
3 rows in set (0.00 sec)

mysql> EXPLAIN EXTENDED SELECT * FROM actor a INNER JOIN film_actor f ON a.`id`=f.`actor_id`;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+--------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+--------------------------------+
|  1 | SIMPLE      | f     | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 |                                |
|  1 | SIMPLE      | a     | ALL  | PRIMARY       | NULL | NULL    | NULL |    4 |    75.00 | Using where; Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+--------------------------------+
2 rows in set, 1 warning (0.00 sec)

2）explain partitions

相比 explain 多了个 partitions 字段，如果查询是基于分区表的话，会显示查询将访问的分区。11
建立表：

DROP TABLE IF EXISTS `hash_part`;

CREATE TABLE `hash_part` (
  `id` INT(11) NOT NULL AUTO_INCREMENT COMMENT '评论ID',
  `comment` VARCHAR(1000) NOT NULL DEFAULT '' COMMENT '评论',
  `ip` VARCHAR(25) NOT NULL DEFAULT '' COMMENT '来源IP',
  PRIMARY KEY (`id`)
) ENGINE=INNODB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8
/*!50100 PARTITION BY HASH (id)
PARTITIONS 3 */;

/*Data for the table `hash_part` */

INSERT  INTO `hash_part`(`id`,`comment`,`ip`) VALUES 

(3,'3','3'),

(1,'1','1'),

(2,'2','2');

/!../ 是一种特殊的注释，其他的数据库产品当然不会执行。mysql特殊处理，会选择性的执行。可以认为是：预编译中的条件编译。特别注意 50100，它表示5.01.00 版本或者更高的版本，才执行。
查看partitions的值：

mysql> explain partitions select * from hash_part where id =1;
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | hash_part | p1         | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain partitions select * from hash_part where id =2;
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | hash_part | p2         | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain partitions select * from hash_part where id =3;
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | hash_part | p0         | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain partitions select * from hash_part where id =4;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-----------------------------------------------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-----------------------------------------------------+
1 row in set (0.00 sec)

1.4 explain中的列

1）id列

select查询的序列号，是一组数字，表示的是查询中执行select子句或者是操作表的顺序。
id的情况有三种，分别是：

id相同表示加载表的顺序是从上到下。
id不同id值越大，优先级越高，越先被执行。
id有相同，也有不同，同时存在。id相同的可以认为是一组，从上往下顺序执行；在所有的组中，id的值越大，优先级越高，越先执行。id为NULL最后执行

mysql> EXPLAIN SELECT f.name,(SELECT id FROM film WHERE id=2) AS id  FROM (SELECT id ,`name` FROM film WHERE id =1) f UNION SELECT `name`,id FROM film;
+----+--------------+------------+--------+---------------+----------+---------+-------+------+-------------+
| id | select_type  | table      | type   | possible_keys | key      | key_len | ref   | rows | Extra       |
+----+--------------+------------+--------+---------------+----------+---------+-------+------+-------------+
|  1 | PRIMARY      | <derived3> | system | NULL          | NULL     | NULL    | NULL  |    1 |             |
|  3 | DERIVED      | film       | const  | PRIMARY       | PRIMARY  | 4       |       |    1 |             |
|  2 | SUBQUERY     | film       | const  | PRIMARY       | PRIMARY  | 4       | const |    1 | Using index |
|  4 | UNION        | film       | index  | NULL          | idx_name | 33      | NULL  |    3 | Using index |
| NULL | UNION RESULT | <union1,4> | ALL    | NULL          | NULL     | NULL    | NULL  | NULL |             |
+----+--------------+------------+--------+---------------+----------+---------+-------+------+-------------+
5 rows in set (0.00 sec)
mysql> explain select (select 1 from actor where id = 1) from (select * from film where id = 1) der;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
|  1 | PRIMARY     | <derived3> | system | NULL          | NULL    | NULL    | NULL  |    1 |             |
|  3 | DERIVED     | film       | const  | PRIMARY       | PRIMARY | 4       |       |    1 |             |
|  2 | SUBQUERY    | actor      | const  | PRIMARY       | PRIMARY | 4       | const |    1 | Using index |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
3 rows in set (0.00 sec)

2）select_type列

select_type 表示对应行是简单还是复杂的查询。

a）simple

简单查询。查询不包含子查询和union

mysql> explain select * from film;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
|  1 | SIMPLE      | film  | index | NULL          | idx_name | 33      | NULL |    3 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
1 row in set (0.00 sec)

b）primary

复杂查询中最外层的 select

c）subquery

包含在 select 中的子查询（不在 from 子句中）

d）derived

包含在 from 子句中的子查询。MySQL会将结果存放在一个临时表中，也称为派生表（derived的英文含义）

mysql> explain select (select 1 from actor where id = 1) from (select * from film where id = 1) der;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
|  1 | PRIMARY     | <derived3> | system | NULL          | NULL    | NULL    | NULL  |    1 |             |
|  3 | DERIVED     | film       | const  | PRIMARY       | PRIMARY | 4       |       |    1 |             |
|  2 | SUBQUERY    | actor      | const  | PRIMARY       | PRIMARY | 4       | const |    1 | Using index |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
3 rows in set (0.00 sec)

set session optimizer_switch='derived_merge=off'; #关闭mysql5.7新特性对衍生表的合并优化
set session optimizer_switch='derived_merge=on'; #还原默认配置

e）union

在 union 中的第二个随后的 select

mysql> explain select 1 union all select 1;
+----+--------------+------------+------+---------------+------+---------+------+------+----------------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+--------------+------------+------+---------------+------+---------+------+------+----------------+
|  1 | PRIMARY      | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | No tables used |
|  2 | UNION        | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | No tables used |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL |                |
+----+--------------+------------+------+---------------+------+---------+------+------+----------------+
3 rows in set (0.00 sec)

3）table列

这一列表示 explain 的一行正在访问哪个表。当 from 子句中有子查询时，table列是格式，表示当前查询依赖 id=N 的查询，于是先执行 id=N 的查询。当有 union 时，UNION RESULT 的 table 列的值为<union1,2>，1和2表示参与 union 的 select 行id。

4） type列

这一列表示关联类型或访问类型，即MySQL决定如何查找表中的行，查找数据行记录的大概范围。依次从最优到最差分别为：system > const > eq_ref > ref > range > index > ALL
一般来说，得保证查询达到range级别，最好达到ref

NULL

mysql能够在优化阶段分解查询语句，在执行阶段用不着再访问表或索引。例如：在索引列中选取最小值，可以单独查找索引来完成，不需要在执行时访问表

mysql> explain select max(id) from film;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                        |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
1 row in set (0.00 sec)

const, system

mysql能对查询的某部分进行优化并将其转化成一个常量（可以看show warnings 的结果）。用于 primary key 或 unique key 的所有列与常数比较时，所以表最多有一个匹配行，读取1次，速度比较快。system是 const的特例，表里只有一条元组匹配时为system

mysql> explain extended select * from (select * from film where id = 1) tmp;
+----+-------------+------------+--------+---------------+---------+---------+------+------+----------+-------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref  | rows | filtered | Extra |
+----+-------------+------------+--------+---------------+---------+---------+------+------+----------+-------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL |    1 |   100.00 |       |
|  2 | DERIVED     | film       | const  | PRIMARY       | PRIMARY | 4       |      |    1 |   100.00 |       |
+----+-------------+------------+--------+---------------+---------+---------+------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)

mysql>  show warnings;
+-------+------+------------------------------------------------+
| Level | Code | Message                                        |
+-------+------+------------------------------------------------+
| Note  | 1003 | select '1' AS `id`,'film1' AS `name` from dual |
+-------+------+------------------------------------------------+
1 row in set (0.00 sec)

eq_ref

primary key 或 unique key 索引的所有部分被连接使用，最多只会返回一条符合条件的记录。这可能是在 const 之外最好的联接类型了，简单的 select 查询不会出现这种 type。

mysql> explain select * from film_actor left join film on film_actor.film_id = film.id;
+----+-------------+------------+--------+---------------+---------+---------+-------------------------+------+-------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref                     | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------------------------+------+-------+
|  1 | SIMPLE      | film_actor | ALL    | NULL          | NULL    | NULL    | NULL                    |    3 |       |
|  1 | SIMPLE      | film       | eq_ref | PRIMARY       | PRIMARY | 4       | test.film_actor.film_id |    1 |       |
+----+-------------+------------+--------+---------------+---------+---------+-------------------------+------+-------+
2 rows in set (0.00 sec)

ref

相比 eq_ref，不使用唯一索引，而是使用普通索引或者唯一性索引的部分前缀，索引要和某个值相比较，可能会找到多个符合条件的行。
a）简单 select 查询，name是普通索引（非唯一索引）

mysql>  explain select * from film where name = 'film1';
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key      | key_len | ref   | rows | Extra                    |
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | film  | ref  | idx_name      | idx_name | 33      | const |    1 | Using where; Using index |
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)

b）关联表查询，idx_film_actor_id是film_id和actor_id的联合索引，这里使用到了film_actor的左边前缀film_id部分。

mysql>  explain select film_id from film left join film_actor on film.id = film_actor.film_id;
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+
| id | select_type | table      | type  | possible_keys     | key               | key_len | ref          | rows | Extra       |
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+
|  1 | SIMPLE      | film       | index | NULL              | idx_name          | 33      | NULL         |    3 | Using index |
|  1 | SIMPLE      | film_actor | ref   | idx_film_actor_id | idx_film_actor_id | 4       | test.film.id |    1 | Using index |
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+
2 rows in set (0.00 sec)

range

范围扫描通常出现在 in(), between ,> ,<, >= 等操作中。使用一个索引来检索给定范围的行。

mysql> explain select * from actor where id > 1;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | actor | range | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

index

扫描全索引就能拿到结果，一般是扫描某个二级索引，这种扫描不会从索引树根节点开始快速查找，而是直接对二级索引的叶子节点遍历和扫描，速度还是比较慢的，**这种查询一般为使用覆盖索引，二级索引一般比较小，所占磁盘空间更小，**所以这种通常比ALL快一些。

mysql> explain select * from film;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
|  1 | SIMPLE      | film  | index | NULL          | idx_name | 33      | NULL |    3 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
1 row in set (0.00 sec)

ALL

即全表扫描，扫描你的聚簇索引的所有叶子节点。通常情况下这需要增加索引来进行优化了。

mysql> explain select * from actor;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | actor | ALL  | NULL          | NULL | NULL    | NULL |    4 |       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

5）possible_keys列

这一列显示查询可能使用哪些索引来查找。 explain 时可能出现 possible_keys 有列，而 key 显示 NULL 的情况，这种情况是因为表中数据不多，mysql认为索引对此查询帮助不大，选择了全表查询。如果该列是NULL，则没有相关的索引。在这种情况下，可以通过检查 where 子句看是否可以创造一个适当的索引来提高查询性能，然后用 explain 查看效果。

6）key列

这一列显示mysql实际采用哪个索引来优化对该表的访问。 如果没有使用索引，则该列是 NULL。如果想强制mysql使用或忽视possible_keys列中的索引，在查询中使用 force index、ignore index。

7）key_len列

这一列显示了mysql在索引里使用的字节数，通过这个值可以算出具体使用了索引中的哪些列。举例来说，film_actor的联合索引 idx_film_actor_id 由 film_id 和 actor_id 两个int列组成，并且每个int是4字节。通过结果中的key_len=4可推断出查询使用了第一个列：film_id列来执行索引查找。

mysql> explain select * from film_actor where film_id = 2;
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------+
| id | select_type | table      | type | possible_keys     | key               | key_len | ref   | rows | Extra |
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------+
|  1 | SIMPLE      | film_actor | ref  | idx_film_actor_id | idx_film_actor_id | 4       | const |    1 |       |
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------+
1 row in set (0.00 sec)

key_len计算规则如下：
字符串，char(n)和varchar(n)，5.0.3以后版本中，**n均代表字符数，而不是字节数，**如果是utf-8，一个数字或字母占1个字节，一个汉字占3个字节

char(n)：如果存汉字长度就是 3n 字节
varchar(n)：如果存汉字则长度是 3n + 2 字节，加的2字节用来存储字符串长度，因为 varchar是变长字符串

数值类型

tinyint：1字节
smallint：2字节
int：4字节
bigint：8字节

时间类型

date：3字节
timestamp：4字节
datetime：8字节

如果字段允许为 NULL，需要1字节记录是否为 NULL
索引最大长度是768字节，当字符串过长时，mysql会做一个类似左前缀索引的处理，将前半部分的字符提取出来做索引。

mysql> explain select * from film_actor where film_id = 2 and actor_id =1;
+----+-------------+------------+------+-------------------+-------------------+---------+-------------+------+-------+
| id | select_type | table      | type | possible_keys     | key               | key_len | ref         | rows | Extra |
+----+-------------+------------+------+-------------------+-------------------+---------+-------------+------+-------+
|  1 | SIMPLE      | film_actor | ref  | idx_film_actor_id | idx_film_actor_id | 8       | const,const |    1 |       |
+----+-------------+------------+------+-------------------+-------------------+---------+-------------+------+-------+
1 row in set (0.00 sec)

film_id和actor_id都是int类型，占4个字节，为4+4=8个字节

mysql> explain select * from film where name='tom';
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key      | key_len | ref   | rows | Extra                    |
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | film  | ref  | idx_name      | idx_name | 33      | const |    1 | Using where; Using index |
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)

name字段的长度为10，key_len=3*10+2+1（字段允许为null需要占用一个字节）=33

8） ref列

这一列显示了在key列记录的索引中，表查找值所用到的列或常量，常见的有：const（常量），字段名（例：film.id）

9）rows列

这一列是mysql估计要读取并检测的行数，注意这个不是结果集里的行数。

10） Extra列

这一列展示的是额外信息。常见的重要值如下：

a）Using index：使用覆盖索引

覆盖索引定义：mysql执行计划explain结果里的key有使用索引，如果select后面查询的字段都可以从这个索引的树中获取，这种情况一般可以说是用到了覆盖索引，extra里一般都有using index；覆盖索引一般针对的是辅助索引，整个查询结果只通过辅助索引就能拿到结果，不需要通过辅助索引树找到主键，再通过主键去主键索引树里获取其它字段值

mysql> explain select film_id from film_actor where film_id = 1;
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------------+
| id | select_type | table      | type | possible_keys     | key               | key_len | ref   | rows | Extra       |
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | film_actor | ref  | idx_film_actor_id | idx_film_actor_id | 4       | const |    2 | Using index |
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------------+
1 row in set (0.00 sec)

b）Using where

使用 where 语句来处理结果，并且查询的列未被索引覆盖

mysql>  explain select * from actor where name = 'a';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | actor | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

c）Using index condition

查询的列不完全被索引覆盖，where条件中是一个前导列的范围；

mysql> explain select * from film_actor where film_id > 1;
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+------+------+----------+-----------------------+
| id | select_type | table      | partitions | type  | possible_keys     | key               | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | film_actor | NULL       | range | idx_film_actor_id | idx_film_actor_id | 4       | NULL |    1 |   100.00 | Using index condition |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

d）Using temporary

mysql需要创建一张临时表来处理查询。出现这种情况一般是要进行优化的，首先是想到用索引来优化。
1） actor.name没有索引，此时创建了张临时表来distinct

mysql> explain select distinct name from actor;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | SIMPLE      | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)

2） film.name建立了idx_name索引，此时查询时extra是using index,没有用临时表

mysql> explain select distinct name from film;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key      | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | film  | NULL       | index | idx_name      | idx_name | 33      | NULL |    3 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

e）Using filesort

将用外部排序而不是索引排序，数据较小时从内存排序，否则需要在磁盘完成排序。这种情况下一般也是要考虑使用索引来优化的。
1. actor.name未创建索引，会浏览actor整个表，保存排序关键字name和对应的id，然后排序name并检索行记录

mysql>  explain select * from actor order by name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

film.name建立了idx_name索引,此时查询时extra是using index

mysql>  explain select * from film order by name;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key      | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | film  | NULL       | index | NULL          | idx_name | 33      | NULL |    3 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

f）Select tables optimized away

使用某些聚合函数（比如 max、min）来访问存在索引的某个字段时

mysql> explain select min(id) from film;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| 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 | Select tables optimized away |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set, 1 warning (0.00 sec)

二、索引最佳实践

表的创建

CREATE TABLE `employees` (
  `id` INT (11) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR (24) NOT NULL DEFAULT '' COMMENT '姓名',
  `age` INT (11) NOT NULL DEFAULT '0' COMMENT '年龄',
  `position` VARCHAR (20) NOT NULL DEFAULT '' COMMENT '职位',
  `hire_time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间',
  PRIMARY KEY (`id`),
  KEY `idx_name_age_position` (`name`, `age`, `position`) USING BTREE
) ENGINE = INNODB AUTO_INCREMENT = 4 DEFAULT CHARSET = utf8 COMMENT = '员工记录表' ;

 INSERT INTO employees(NAME,age,POSITION,hire_time) VALUES('LiLei',22,'manager',NOW()); 
 INSERT INTO employees(NAME,age,POSITION,hire_time) VALUES('HanMeimei', 23,'dev',NOW()); 
 INSERT INTO employees(NAME,age,POSITION,hire_time) VALUES('Lucy',23,'dev',NOW());

2.1 全值匹配

mysql> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei';
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys         | key                   | key_len | ref   | rows | filtered | Extra |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | employees | NULL       | ref  | idx_name_age_position | idx_name_age_position | 74      | const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22;
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys         | key                   | key_len | ref         | rows | filtered | Extra |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | employees | NULL       | ref  | idx_name_age_position | idx_name_age_position | 78      | const,const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22 AND position ='manager';
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys         | key                   | key_len | ref               | rows | filtered | Extra |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | employees | NULL       | ref  | idx_name_age_position | idx_name_age_position | 140     | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

74：name的长度=3n+2=324+2=74
78：name的长度+age的长度=（324+2）+4=78
140：name的长度+age的长度+position的长度=（324+2）+4+（203+2）=140

2.2 最左前缀法则

如果索引了多列，要遵守最左前缀法则。指的是查询从索引的最左前列开始并且不跳过索引中的列。

mysql> EXPLAIN SELECT * FROM employees WHERE name = 'Bill' and age = 31;
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys         | key                   | key_len | ref         | rows | filtered | Extra |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | employees | NULL       | ref  | idx_name_age_position | idx_name_age_position | 78      | const,const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE age = 30 AND position = 'dev';
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | employees | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE position = 'manager';
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | employees | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

2.3 不在索引列做计算

不在索引列上做任何操作（计算、函数、（自动or手动）类型转换），会导致索引失效而转向全表扫描

mysql> EXPLAIN SELECT * FROM employees WHERE name = 'LiLei';
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys         | key                   | key_len | ref   | rows | filtered | Extra |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | employees | NULL       | ref  | idx_name_age_position | idx_name_age_position | 74      | const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql>  EXPLAIN SELECT * FROM employees WHERE left(name,3) = 'LiLei';
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | employees | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

给hire_time增加一个普通索引：

mysql> ALTER TABLE `employees` ADD INDEX `idx_hire_time` (`hire_time`) USING BTREE ;
Query OK, 0 rows affected (0.22 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> EXPLAIN select * from employees where date(hire_time) ='2018‐09‐30';
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | employees | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 2 warnings (0.00 sec)

转化为日期范围查询，有可能会走索引：

mysql>  EXPLAIN select * from employees where hire_time >='2018-09-30 00:00:00' and hire_time <='2018-09-30 23:59:59';
+----+-------------+-----------+------------+-------+---------------+---------------+---------+------+------+----------+-----------------------+
| id | select_type | table     | partitions | type  | possible_keys | key           | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-----------+------------+-------+---------------+---------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | employees | NULL       | range | idx_hire_time | idx_hire_time | 4       | NULL |    1 |   100.00 | Using index condition |
+----+-------------+-----------+------------+-------+---------------+---------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

还原索引的状态：

ALTER TABLE `employees` DROP INDEX `idx_hire_time`;

2.4 存储引擎不能使用索引中范围条件右边的列

mysql> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22 AND position ='manager';
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys         | key                   | key_len | ref               | rows | filtered | Extra |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | employees | NULL       | ref  | idx_name_age_position | idx_name_age_position | 140     | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age > 22 AND position ='manage r';
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table     | partitions | type  | possible_keys         | key                   | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | employees | NULL       | range | idx_name_age_position | idx_name_age_position | 78      | NULL |    1 |    33.33 | Using index condition |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

2.5 尽量使用覆盖索引（只访问索引的查询（索引列包含查询列）），减少 select * 语句

mysql> EXPLAIN SELECT name,age FROM employees WHERE name= 'LiLei' AND age = 23 AND position ='manager';
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys         | key                   | key_len | ref               | rows | filtered | Extra       |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------------+
|  1 | SIMPLE      | employees | NULL       | ref  | idx_name_age_position | idx_name_age_position | 140     | const,const,const |    1 |   100.00 | Using index |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 23 AND position ='manager';
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys         | key                   | key_len | ref               | rows | filtered | Extra |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | employees | NULL       | ref  | idx_name_age_position | idx_name_age_position | 140     | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

2.6 mysql在使用不等于（！=或者<>），not in ，not exists 的时候无法使用索引会导致全表扫描

< 小于、 > 大于、 <=、>= 这些，mysql内部优化器会根据检索比例、表大小等多个因素整体评估是否使用索引

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.5.40    |
+-----------+
1 row in set (0.01 sec)

mysql>  EXPLAIN SELECT * FROM employees WHERE name != 'LiLei';
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
| id | select_type | table     | type | possible_keys         | key  | key_len | ref  | rows | Extra       |
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | employees | ALL  | idx_name_age_position | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE name not in( 'LiLei');
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
| id | select_type | table     | type | possible_keys         | key  | key_len | ref  | rows | Extra       |
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | employees | ALL  | idx_name_age_position | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql8的情况下会走索引：

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.11    |
+-----------+
1 row in set (0.00 sec)

mysql>  EXPLAIN SELECT * FROM employees WHERE name != 'LiLei';
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table     | partitions | type  | possible_keys         | key                   | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | employees | NULL       | range | idx_name_age_position | idx_name_age_position | 74      | NULL |    2 |   100.00 | Using index condition |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE name not in( 'LiLei');
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table     | partitions | type  | possible_keys         | key                   | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | employees | NULL       | range | idx_name_age_position | idx_name_age_position | 74      | NULL |    2 |   100.00 | Using index condition |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

2.7 is null,is not null 一般情况下也无法使用索引

mysql> EXPLAIN SELECT * FROM employees WHERE name is null;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE name is not null;
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
| id | select_type | table     | type | possible_keys         | key  | key_len | ref  | rows | Extra       |
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | employees | ALL  | idx_name_age_position | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

2.8 like以通配符开头（‘$abc…’）mysql索引失效会变成全表扫描操作

mysql> EXPLAIN SELECT * FROM employees WHERE name like '%Lei';
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | employees | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE name like 'Lei%';
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table     | partitions | type  | possible_keys         | key                   | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | employees | NULL       | range | idx_name_age_position | idx_name_age_position | 74      | NULL |    1 |   100.00 | Using index condition |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

问题：解决like’%字符串%'索引不被使用的方法？
a）使用覆盖索引，查询字段必须是建立覆盖索引字段

mysql> EXPLAIN SELECT name,age,position FROM employees WHERE name like '%Lei%';
+----+-------------+-----------+------------+-------+---------------+-----------------------+---------+------+------+----------+--------------------------+
| id | select_type | table     | partitions | type  | possible_keys | key                   | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-----------+------------+-------+---------------+-----------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | employees | NULL       | index | NULL          | idx_name_age_position | 140     | NULL |    3 |    33.33 | Using where; Using index |
+----+-------------+-----------+------------+-------+---------------+-----------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

b）如果不能使用覆盖索引则可能需要借助搜索引擎

2.9 字符串不加单引号索引失效

mysql> EXPLAIN SELECT * FROM employees WHERE name = '1000';
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys         | key                   | key_len | ref   | rows | filtered | Extra |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | employees | NULL       | ref  | idx_name_age_position | idx_name_age_position | 74      | const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> EXPLAIN SELECT * FROM employees WHERE name = 1000;
+----+-------------+-----------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys         | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-----------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | employees | NULL       | ALL  | idx_name_age_position | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-----------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set, 3 warnings (0.00 sec)

2.10 少用or或in，用它查询时，mysql不一定使用索引，mysql内部优化器会根据检索比例、表大小等多个因素整体评估是否使用索引

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.5.40    |
+-----------+
1 row in set (0.00 sec)

mysql>  EXPLAIN SELECT * FROM employees WHERE name = 'LiLei' or name = 'HanMeimei';
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
| id | select_type | table     | type | possible_keys         | key  | key_len | ref  | rows | Extra       |
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | employees | ALL  | idx_name_age_position | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql8会走索引：

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.11    |
+-----------+
1 row in set (0.00 sec)

mysql>  EXPLAIN SELECT * FROM employees WHERE name = 'LiLei' or name = 'HanMeimei';
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table     | partitions | type  | possible_keys         | key                   | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | employees | NULL       | range | idx_name_age_position | idx_name_age_position | 74      | NULL |    2 |   100.00 | Using index condition |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

2.11 范围查询优化

给年龄添加单值索引

ALTER TABLE `employees` ADD INDEX `idx_age` (`age`) USING BTREE ;

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.5.40    |
+-----------+
1 row in set (0.00 sec)

mysql> explain select * from employees where age >=1 and age <=2000;
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table     | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | employees | ALL  | idx_age       | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

没走索引原因：mysql内部优化器会根据检索比例、表大小等多个因素整体评估是否使用索引。比如这个例子，可能是由于单次数据量查询过大导致优化器最终选择不走索引
优化方法：可以将大的范围拆分成多个小范围

mysql>  explain select * from employees where age >=1 and age <=1000;
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table     | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | employees | ALL  | idx_age       | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from employees where age >=1001 and age <=2000;
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table     | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | employees | range | idx_age       | idx_age | 4       | NULL |    1 | Using where |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql8会走索引：

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.11    |
+-----------+
1 row in set (0.00 sec)

mysql> explain select * from employees where age >=1 and age <=2000;
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | employees | NULL       | range | idx_age       | idx_age | 4       | NULL |    3 |   100.00 | Using index condition |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

还原最初索引状态

ALTER TABLE `employees` DROP INDEX `idx_age`;

2.12 索引使用总结

在这里插入图片描述

like KK%相当于=常量，%KK和%KK% 相当于范围

‐‐ mysql5.7关闭ONLY_FULL_GROUP_BY报错 
select version(), @@sql_mode;SET sql_mode=(SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY',''));

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当前栏目

02-explain详解与索引最佳实践

一、explain的使用和详解

1.1 explain工具介绍

1.2 Explain分析示例

1.3 explain 两个变种

1）explain extended

2）explain partitions

1.4 explain中的列

1）id列

2）select_type列

a）simple

b）primary

c）subquery

d）derived

e）union

3）table列

4） type列

NULL

const, system

eq_ref

ref

range

index

ALL

5）possible_keys列

6）key列

7）key_len列

8） ref列

9）rows列

10） Extra列

a）Using index：使用覆盖索引

b）Using where

c）Using index condition

d）Using temporary

e）Using filesort

f）Select tables optimized away

二、索引最佳实践

2.1 全值匹配

2.2 最左前缀法则

2.3 不在索引列做计算

2.4 存储引擎不能使用索引中范围条件右边的列

2.5 尽量使用覆盖索引（只访问索引的查询（索引列包含查询列）），减少 select * 语句

2.6 mysql在使用不等于（！=或者<>），not in ，not exists 的时候无法使用索引会导致全表扫描

2.7 is null,is not null 一般情况下也无法使用索引

2.8 like以通配符开头（‘$abc…’）mysql索引失效会变成全表扫描操作

2.9 字符串不加单引号索引失效

2.10 少用or或in，用它查询时，mysql不一定使用索引，mysql内部优化器会根据检索比例、表大小等多个因素整体评 估是否使用索引

2.11 范围查询优化

2.12 索引使用总结

相关文章

2.10 少用or或in，用它查询时，mysql不一定使用索引，mysql内部优化器会根据检索比例、表大小等多个因素整体评估是否使用索引