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
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。
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
EXPLAIN SELECT * FROM employees WHERE name = ‘LiLei’;
EXPLAIN SELECT * FROM employees WHERE left(name,3) = ‘LiLei’;
给hire_time增加一个普通索引：
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 = 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<>）的时候无法使用索引会导致全表扫描
EXPLAIN SELECT * FROM employees WHERE name != ‘LiLei’;
7.is null,is not null 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%
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%’;
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
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
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：可以讲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
还原最初索引状态
1ALTER TABLE employees
2DROP INDEX idx_age;
索引使用总结：
like KK%相当于=常量，%KK和%KK% 相当于范围