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());

• 创建了 name, age, position 联合索引

全值匹配

1. 示例一

EXPLAIN SELECT * FROM employees WHERE name = 'LiLei';

• 结果

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ref	idx_name_age_position	idx_name_age_position	74	const	1	100	null

2. 示例 2

EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22;

• 结果

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ref	idx_name_age_position	idx_name_age_position	78	const,const	1	100	null

3. 示例 3

EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22 AND position='manager'

• 结果

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ref	idx_name_age_position	idx_name_age_position	140	const,const,const	1	100	null

最左前缀法则

如果索引创建了联合索引， 那么就必须遵守最左前缀索引。具体指的就是查询从索引的最左列开始并不跳过索引中的列

1. 案例一

EXPLAIN SELECT * FROM employees WHERE name = 'Bill' and age = 31;

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ref	idx_name_age_position	idx_name_age_position	78	const,const	1	100	null

• 备注： 走索引

2. 案例二

EXPLAIN SELECT * FROM employees WHERE age = 30 AND position = 'dev';

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ALL					3	33.33	null

• 备注： 索引失效

3. 案例 3

EXPLAIN SELECT * FROM employees WHERE position = 'manager';

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ALL					3	33.33	null

• 备注：索引失效

不在索引上做任何操作（计算、函数、类型转换），会导致索引失效转向全表扫描

EXPLAIN SELECT * FROM employees WHERE left(name,3) = 'LiLei';

存储引擎不能使用索引中范围条件右边的列

 EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 22 AND position ='manager';

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ref	idx_name_age_position	idx_name_age_position	140	const,const	1	100	null

 EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age > 22 AND position ='manage
r';

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	range	idx_name_age_position	idx_name_age_position	78		1	100	null

• 备注： 使用了范围查询，从 key_len 可以知道 position 的索引不生效

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

EXPLAIN SELECT name,age FROM employees WHERE name= 'LiLei' AND age = 23 AND position
='manager';

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ref	idx_name_age_position	idx_name_age_position	140	const,const,const	1	100	Using index

EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age = 23 AND position
='manager';

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ref	idx_name_age_position	idx_name_age_position	140	const,const,const	1	100

• 备注： Extra 的区别

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

EXPLAIN SELECT * FROM employees WHERE name != 'LiLei';

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ALL	idx_name_age_position				3	66.67	Using where

is null,is not null 一般情况下也无法使用索引

EXPLAIN SELECT * FROM employees WHERE name is null

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE										Impossible WHERE

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

EXPLAIN SELECT * FROM employees WHERE name like 'Lei%'

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	range	idx_name_age_position	idx_name_age_position	74		1	100	Using index condition

EXPLAIN SELECT * FROM employees WHERE name like '%Lei'

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ALL					3	33.33	Using where

• 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

EXPLAIN SELECT name,age,position FROM employees WHERE name like '%Lei%';

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	index		idx_name_age_position	140		3	33.33	Using where index

b. 如果不能使用覆盖索引则可能需要借助搜索引擎

字符串不加单引号索引失效

EXPLAIN SELECT * FROM employees WHERE name = '1000';
EXPLAIN SELECT * FROM employees WHERE name = 1000;

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

EXPLAIN SELECT * FROM employees WHERE name = 'LiLei' or name = 'HanMeimei';

id	select_type	table	partitions	type	possible_keys	key	ken_len	ref	rows	filtered	Extra
1	SIMPLE	employee	null	ALL	idx_name_age_position				3	66.67	Using where

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 ;
1 explain select * from employees where age >=1 and age <=2000;

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

explain select * from employees where age >=1 and age <=1000;
explain select * from employees where age >=1001 and age <=2000;

索引使用总结

假设 index(a,b,c)

Where 语句	索引是否被使用
where a= 3	Y，使用到 a
where a= 3 and b =5	Y,使用到 a,b
where a = 3 and b =5 and c = 4	Y,使用 a,b,c
where b = 3 或者 where b=3 and c=4 或者 c =4	N
where a = 3 and c= 5	使用到 a, c 不可用，b 中断了
where a = 3 and b >4 and c =5	使用到 a,b, c 不能用在范围之后，b 断了
where a = 3 and b like 'kk%' and c =4	Y, 使用到 a,b,b
where a=3 and b like '%kk' and c=4	Y,只使用到 a
where a = 3 and b like '%kk%' and c=4	Y,使用到 a
where a= 3 and b like 'k%kk' and c= 4	Y，使用到了 a,b,c

like kk%相当于=常量，%kk 和%kk%相当于范围