MySQL索引最佳实践

MySQL索引优化和最佳实践

1. 索引实践

创建表单

 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());
INSERT INTO employees(name,age,position,hire_time) VALUES('swj',27,'dev',NOW());

1. 全值匹配

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

EXPLAIN SELECT * FROM employees WHERE name= 'swj' AND age = 27;

EXPLAIN SELECT * FROM employees WHERE name= 'swj' AND age = 27 and position ='dev'

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。

1 EXPLAIN SELECT * FROM employees WHERE name = 'Bill' and age = 31;
2 EXPLAIN SELECT * FROM employees WHERE age = 30 AND position = 'dev';
3 EXPLAIN SELECT * FROM employees WHERE position = 'manager';

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

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

4. 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';
EXPLAIN SELECT * FROM employees WHERE name= 'LiLei' AND age > 22 AND position ='manager';

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

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

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

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

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

7. 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

8. 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'

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

9. 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; 

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

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

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 ;
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

还原最初索引状态

ALTER TABLE `employees` DROP INDEX `idx_age`;

2. 常见问题与思考

1. 解决like’%字符串%'索引不被使用的方法？

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%';

如果不能使用覆盖索引则可能需要借助搜索引擎

3.索引使用总结

like KK%相当于=常量，%KK和%KK% 相当于范围