Explain详解与索引最佳实践

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

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

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;

INSERT INTO `actor` (`id`, `name`, `update_time`) VALUES (1,'a','2017-12-22 15:27:18'), (2,'b','2017-12-22 15:27:18'), (3,'c','2017-12-22 15:27:18');


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 DEFAULT CHARSET=utf8;

INSERT INTO `film` (`id`, `name`) VALUES (3,'film0'),(1,'film1'),(2,'film2');


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;

INSERT INTO `film_actor` (`id`, `film_id`, `actor_id`) VALUES (1,1,1),(2,1,2),(3,2,1);

MariaDB [test]> 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 (0.003 sec)

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，是一个半分比的值，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）。

MariaDB [test]> 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 (0.002 sec)

MariaDB [test]> show warnings;
+-------+------+---------------------------------------------------------------+
| Level | Code | Message                                                       |
+-------+------+---------------------------------------------------------------+
| Note  | 1003 | select 1 AS `id`,'film1' AS `name` from `test`.`film` where 1 |
+-------+------+---------------------------------------------------------------+
1 row in set (0.001 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。

explain中的列

接下来我们将展示 explain 中每个列的信息。

MariaDB [test]> explain select * from film where id = 2;
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id   | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|    1 | SIMPLE      | film  | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.003 sec)

1. id列

id列的编号是 select 的序列号，有几个 select 就有几个id，并且id的顺序是按 select 出现的顺序增长的。

id列越大执行优先级越高，id相同则从上往下执行，id为NULL最后执行。

2. select_type列

select_type 表示对应行是简单还是复杂的查询。

1）simple：简单查询。查询不包含子查询和union。

MariaDB [test]> explain select * from film where id = 2;
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id   | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|    1 | SIMPLE      | film  | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.003 sec)

2）primary：复杂查询中最外层的 select

3）subquery：包含在 select 中的子查询（不在 from 子句中）

4）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）

用这个例子来了解 primary、subquery 和 derived 类型

MariaDB [test]> set session optimizer_switch='derived_merge=off'; #关闭mysql5.7新特性对衍生表的合并优化
Query OK, 0 rows affected (0.001 sec)

MariaDB [test]> 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       | const |    1 |             |
|    2 | SUBQUERY    | actor      | const  | PRIMARY       | PRIMARY | 4       | const |    1 | Using index |
+------+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
3 rows in set (0.005 sec)

set session optimizer_switch='derived_merge=on'; # 还原默认配置

5）union：在 union 中的第二个和随后的 select

MariaDB [test]> 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 |
+------+-------------+-------+------+---------------+------+---------+------+------+----------------+
2 rows in set (0.002 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。

MariaDB [test]> explain select min(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 (0.002 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。

MariaDB [test]>  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 | SIMPLE      | film  | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 |       |
+------+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.004 sec)

MariaDB [test]> show warnings;
+-------+------+---------------------------------------------------------------+
| Level | Code | Message                                                       |
+-------+------+---------------------------------------------------------------+
| Note  | 1003 | select 1 AS `id`,'film1' AS `name` from `test`.`film` where 1 |
+-------+------+---------------------------------------------------------------+
1 row in set (0.001 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。

MariaDB [test]> 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 (0.002 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。

1. 简单 select 查询，name是普通索引（非唯一索引）

MariaDB [test]> 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 (0.004 sec)

2.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。

MariaDB [test]> 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              | PRIMARY           | 4       | 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 (0.003 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。

MariaDB [test]> 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 |    2 | Using where |
+------+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.003 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，所以这种通常比ALL快一些。

MariaDB [test]> 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 (0.002 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。

MariaDB [test]> 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 (0.002 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。

MariaDB [test]> 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 (0.003 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。

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：

1）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

MariaDB [test]> 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 (0.003 sec)

2）Using where：使用 where 语句来处理结果，并且查询的列未被索引覆盖

MariaDB [test]> 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 |    3 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.003 sec)

3）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；

MariaDB [test]> explain select * from film_actor where film_id > 1;
+------+-------------+------------+-------+-------------------+-------------------+---------+------+------+-----------------------+
| id   | select_type | table      | type  | possible_keys     | key               | key_len | ref  | rows | Extra                 |
+------+-------------+------------+-------+-------------------+-------------------+---------+------+------+-----------------------+
|    1 | SIMPLE      | film_actor | range | idx_film_actor_id | idx_film_actor_id | 4       | NULL |    1 | Using index condition |
+------+-------------+------------+-------+-------------------+-------------------+---------+------+------+-----------------------+
1 row in set (0.003 sec)

4）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

MariaDB [test]> explain select distinct name from actor;
+------+-------------+-------+------+---------------+------+---------+------+------+-----------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra           |
+------+-------------+-------+------+---------------+------+---------+------+------+-----------------+
|    1 | SIMPLE      | actor | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using temporary |
+------+-------------+-------+------+---------------+------+---------+------+------+-----------------+
1 row in set (0.003 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

MariaDB [test]> explain select distinct name 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 (0.003 sec)

5）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

MariaDB [test]> explain select * from actor order by name;
+------+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+------+-------------+-------+------+---------------+------+---------+------+------+----------------+
|    1 | SIMPLE      | actor | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using filesort |
+------+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.003 sec)

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

MariaDB [test]> explain select * from film order by name;
+------+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 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 (0.003 sec)

6）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是

MariaDB [test]> explain select min(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 (0.003 sec)

索引最佳实践

# 示例表：
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());

1.全值匹配

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei';
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+
| id   | select_type | table     | type | possible_keys         | key                   | key_len | ref   | rows | Extra                 |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+
|    1 | SIMPLE      | employees | ref  | idx_name_age_position | idx_name_age_position | 74      | const |    1 | Using index condition |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+
1 row in set (0.002 sec)

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22;
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+
| id   | select_type | table     | type | possible_keys         | key                   | key_len | ref         | rows | Extra                 |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+
|    1 | SIMPLE      | employees | ref  | idx_name_age_position | idx_name_age_position | 78      | const,const |    1 | Using index condition |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+
1 row in set (0.005 sec)

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。

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name = 'Bill' and age = 31;
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+
| id   | select_type | table     | type | possible_keys         | key                   | key_len | ref         | rows | Extra                 |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+
|    1 | SIMPLE      | employees | ref  | idx_name_age_position | idx_name_age_position | 78      | const,const |    1 | Using index condition |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------+------+-----------------------+
1 row in set (0.004 sec)

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE age = 30 AND position = 'dev';
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table     | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | employees | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.003 sec)

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE position = 'manager';
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table     | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | employees | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.002 sec)

3.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

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name = 'LiLei';
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+
| id   | select_type | table     | type | possible_keys         | key                   | key_len | ref   | rows | Extra                 |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+
|    1 | SIMPLE      | employees | ref  | idx_name_age_position | idx_name_age_position | 74      | const |    1 | Using index condition |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+
1 row in set (0.003 sec)

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE left(name,3) = 'LiLei';
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table     | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | employees | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.003 sec)

给hire_time增加一个普通索引：

MariaDB [test]> ALTER TABLE `employees` ADD INDEX `idx_hire_time` (`hire_time`) USING BTREE ;
Query OK, 0 rows affected (0.046 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> EXPLAIN select * from employees where date(hire_time) ='2018‐09‐30';
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table     | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | employees | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.003 sec)

转化为日期范围查询，有可能会走索引：

MariaDB [test]> 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     | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | employees | ALL  | idx_hire_time | NULL | NULL    | NULL |    3 | Using where |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.003 sec)

还原最初索引状态

ALTER TABLE employees DROP INDEX idx_hire_time;                                               

4.存储引擎不能使用索引中范围条件右边的列

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22 AND position ='manager';
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+
| id   | select_type | table     | type | possible_keys         | key                   | key_len | ref               | rows | Extra                 |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+
|    1 | SIMPLE      | employees | ref  | idx_name_age_position | idx_name_age_position | 140     | const,const,const |    1 | Using index condition |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+
1 row in set (0.004 sec)

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age > 22 AND position ='manager';
+------+-------------+-----------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+
| id   | select_type | table     | type  | possible_keys         | key                   | key_len | ref  | rows | Extra                 |
+------+-------------+-----------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+
|    1 | SIMPLE      | employees | range | idx_name_age_position | idx_name_age_position | 78      | NULL |    1 | Using index condition |
+------+-------------+-----------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+
1 row in set (0.004 sec)

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

MariaDB [test]> EXPLAIN SELECT name,age FROM employees WHERE name= 'LiLei' AND age = 23 AND position='manager';
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------------+------+--------------------------+
| id   | select_type | table     | type | possible_keys         | key                   | key_len | ref               | rows | Extra                    |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------------+------+--------------------------+
|    1 | SIMPLE      | employees | ref  | idx_name_age_position | idx_name_age_position | 140     | const,const,const |    1 | Using where; Using index |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------------+------+--------------------------+
1 row in set (0.003 sec)

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 23 AND position ='manager';
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+
| id   | select_type | table     | type | possible_keys         | key                   | key_len | ref               | rows | Extra                 |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+
|    1 | SIMPLE      | employees | ref  | idx_name_age_position | idx_name_age_position | 140     | const,const,const |    1 | Using index condition |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------------------+------+-----------------------+
1 row in set (0.003 sec)

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

MariaDB [test]> 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 (0.003 sec)

7.is null,is not null 一般情况下也无法使用索引

MariaDB [test]> 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 (0.002 sec)

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

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name like '%Lei';
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table     | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | employees | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where |
+------+-------------+-----------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.003 sec)

MariaDB [test]>
MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name like 'Lei%';
+------+-------------+-----------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+
| id   | select_type | table     | type  | possible_keys         | key                   | key_len | ref  | rows | Extra                 |
+------+-------------+-----------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+
|    1 | SIMPLE      | employees | range | idx_name_age_position | idx_name_age_position | 74      | NULL |    1 | Using index condition |
+------+-------------+-----------+-------+-----------------------+-----------------------+---------+------+------+-----------------------+
1 row in set (0.004 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

MariaDB [test]> EXPLAIN SELECT name,age,position FROM employees WHERE name like '%Lei%';
+------+-------------+-----------+-------+---------------+-----------------------+---------+------+------+--------------------------+
| id   | select_type | table     | type  | possible_keys | key                   | key_len | ref  | rows | Extra                    |
+------+-------------+-----------+-------+---------------+-----------------------+---------+------+------+--------------------------+
|    1 | SIMPLE      | employees | index | NULL          | idx_name_age_position | 140     | NULL |    3 | Using where; Using index |
+------+-------------+-----------+-------+---------------+-----------------------+---------+------+------+--------------------------+
1 row in set (0.003 sec)

b）如果不能使用覆盖索引则可能需要借助搜索引擎

9.字符串不加单引号索引失效

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name = '1000';
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+
| id   | select_type | table     | type | possible_keys         | key                   | key_len | ref   | rows | Extra                 |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+
|    1 | SIMPLE      | employees | ref  | idx_name_age_position | idx_name_age_position | 74      | const |    1 | Using index condition |
+------+-------------+-----------+------+-----------------------+-----------------------+---------+-------+------+-----------------------+
1 row in set (0.002 sec)

MariaDB [test]> EXPLAIN SELECT * FROM employees WHERE name = 1000;
+------+-------------+-----------+------+-----------------------+------+---------+------+------+-------------+
| 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 (0.003 sec)

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，详见范围查询优化

MariaDB [test]> 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 (0.003 sec)

11.范围查询优化

给年龄添加单值索引

MariaDB [test]> ALTER TABLE `employees` ADD INDEX `idx_age` (`age`) USING BTREE;
Query OK, 0 rows affected (0.040 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> 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 (0.004 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

MariaDB [test]> 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 (0.003 sec)

MariaDB [test]> 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 index condition |
+------+-------------+-----------+-------+---------------+---------+---------+------+------+-----------------------+
1 row in set (0.003 sec)

还原最初索引状态

ALTER TABLE `employees` DROP INDEX `idx_age`;

索引使用总结

like KK%相当于=常量，%KK和%KK% 相当于范围