MySQL调优（二）——索引最佳实践

1.全值匹配
EXPLAIN SELECT * FROM employees WHERE name= ‘LiLei’;

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

EXPLAIN SELECT * FROM employees WHERE name= ‘LiLei’ AND age = 22 ANDposition =‘manager’;

2.最左前缀法则
若是索引了多列，要遵照最左前缀法则。
指的是查询从索引的最左前列开始而且不跳过索引中的列。
EXPLAIN SELECT * FROM employees WHERE age = 22 AND position =‘manager’;
EXPLAIN SELECT * FROM employees WHERE position = ‘manager’;
EXPLAIN SELECT * FROM employees WHERE name = ‘LiLei’;

3.不在索引列上作任何操做（计算、函数、（自动or手动）类型转换），会致使索引失效而转向全表扫描
EXPLAIN SELECT * FROM employees WHERE name = ‘LiLei’;
EXPLAIN SELECT * FROM employees WHERE left(name,3) = ‘LiLei’;
给hire_time增长一个普通索引：

EXPLAIN select * from employees where date(hire_time) =‘2018-09-30’

EXPLAIN select * from employees where hire_time >=‘2018-09-30 00:00:00’ andhire_time <=‘2018-09-30 23:59:59’

还原最初索引状态

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 ANDposition =‘manager’;
EXPLAIN SELECT * FROM employees WHERE name= ‘LiLei’ AND age > 22 ANDposition =‘manager’

5.尽可能使用覆盖索引（只访问索引的查询（索引列包含查询列）），减小select *语句
EXPLAIN SELECT name,age FROM employees WHERE name= ‘LiLei’ AND age = 23AND position =‘manager

EXPLAIN SELECT * FROM employees WHERE name= ‘LiLei’ AND age = 23 ANDposition =‘manager’

6.mysql在使用不等于（！=或者<>）的时候没法使用索引会致使全表扫描
EXPLAIN SELECT * FROM employees WHERE name != ‘LiLei’;

7.is null,is not null 也没法使用索引
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做
EXPLAIN SELECT * FROM employees WHERE name like ‘%Lei’

EXPLAIN SELECT * FROM employees WHERE name like ‘Lei%

问题：解决like’%字符串%'索引不被使用的方法？
a）使用覆盖索引，查询字段必须是创建覆盖索引字段
EXPLAIN SELECT name,age,position FROM employees WHERE name like ‘%Lei%’;

b）若是不能使用覆盖索引则可能须要借助搜索引擎mysql

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不必定使用索引，mysql内部优化器会根据检索比例、表大小等多个因素总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

explain select * from employees where age >=1 and age <=2000;
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没走索引缘由：mysql内部优化器会根据检索比例、表大小等多个因素总体评估是否使用索引。
好比这个例子，多是因为单次数据量查询过大致使优化器最终选择不走索引优化方法：能够讲大的范围拆分红多个小范围
explain select * from employees where age >=1 and age <=1000;
explain select * from employees where age >=1001 and age <=2000

还原最初索引状态
1ALTER TABLE employees
2DROP INDEX idx_age;
索引使用总结：

like KK%至关于=常量，%KK和%KK% 至关于范围sql