二.Explain详解与索引最佳实践_二、explain详解与索引最佳实践-优快云博客

本文链接：https://blog.youkuaiyun.com/wangxuekuicsdn/article/details/119754527

1 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);

mysql> explain select * 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 |    2 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.41 sec)

1.explain中的列

接下来我们将展示 explain 中每个列的信息。

(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：

1）simple：简单查询。查询不包含子查询和union

mysql> explain select * from film where id =2;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | film  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 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mysql> set session

mysql> set session optimizer_switch='derived_merge=off';
Query OK, 0 rows affected (0.34 sec)

mysql>  explain select (select 1 from actor where id = 1) from (select * from film where
    -> id = 1) der;
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table      | partitions | type   | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | <derived3> | NULL       | system | NULL          | NULL    | NULL    | NULL  |    1 |   100.00 | NULL        |
|  3 | DERIVED     | film       | NULL       | const  | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL        |
|  2 | SUBQUERY    | actor      | NULL       | const  | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)

mysql>  set session optimizer_switch='derived_merge=on';
Query OK, 0 rows affected (0.00 sec)

5）union：在 union 中的第二个和随后的 select

mysql> explain select 1 union all select 1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | PRIMARY     | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used |
|  2 | UNION       | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+

（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，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 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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+

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 select * from (select * from film where id = 1) tmp;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | film  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (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      | partitions | type   | possible_keys | key     | key_len | ref                           | rows | filtered | Extra |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------------------------------+------+----------+-------+
|  1 | SIMPLE      | film_actor | NULL       | ALL    | NULL          | NULL    | NULL    | NULL                          |    3 |   100.00 | NULL  |
|  1 | SIMPLE      | film       | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | mysqlstudy.film_actor.film_id |    1 |   100.00 | NULL  |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------------------------------+------+----------+-------+
2 rows in set, 1 warning (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。

1. 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 | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | film  | NULL       | ref  | idx_name      | idx_name | 33      | const |    1 |   100.00 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 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。

mysql>  explain select film_id from film left join film_actor on film.id = film_actor.film_id;
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+--------------------+------+----------+-------------+
| 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 | SIMPLE      | film_actor | NULL       | ref   | idx_film_actor_id | idx_film_actor_id | 4       | mysqlstudy.film.id |    1 |   100.00 | Using index |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+--------------------+------+----------+-------------+
2 rows in set, 1 warning (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 | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | actor | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    2 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (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，所以这种通常比ALL快一些。

mysql> explain select * from film;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| 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)

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 | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (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

（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      | partitions | type | possible_keys     | key               | key_len | ref   | rows | filtered | Extra       |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | film_actor | NULL       | ref  | idx_film_actor_id | idx_film_actor_id | 4       | const |    2 |   100.00 | Using index |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (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 | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |    50.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (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。

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 |    2 |   100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
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,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 (0.00 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 |    2 |   100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.13 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是

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 (0.66 sec)

2.索引最佳实战

1 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',
,'dev',NOW());
 INSERT INTO employees(name,age,position,hire_time) VALUES('Lucy',23,'dev',NOW());

（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  |
+----+-------------+-----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+

（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。

KEY `idx_name_age_position` (`name`,`age`,`position`) USING BTREE

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 | 97324 |     1.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 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 | 97324 |    10.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 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

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 | 97324 |   100.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

(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 ='manager';
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| 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 |    10.00 | Using index condition |
+----+-------------+-----------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 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

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

7.is null,is not null 一般情况下也无法使用索引

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 | 97324 |    11.11 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.47 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 (0.34 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 | 97324 |    11.11 | 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

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 | 97324 |    10.00 | Using where |
+----+-------------+-----------+------------+------+-----------------------+------+---------+------+-------+----------+-------------+
1 row in set, 3 warnings (0.35 sec)

3.索引使用总结