Mysql索引笔记

订单表，tb_order表，索引如下两种：
KEY `idx_created_at` (`created_at`) USING BTREE,
KEY `idx_distribution_id_pay_at` (`distribution_id`,`pay_at`),
  
EXPLAIN select * from tb_order WHERE  created_at>=NOW() ORDER BY created_at;命中索引
EXPLAIN select * from tb_order WHERE  created_at<=NOW() ORDER BY created_at desc LIMIT 20;命中索引
EXPLAIN select * from tb_order WHERE  created_at<=NOW() ORDER BY created_at desc;没有命中索引

EXPLAIN select * from tb_order ORDER BY created_at LIMIT 20;命中索引
EXPLAIN select * from tb_order ORDER BY created_at;没有命中索引

EXPLAIN select * from tb_order WHERE pay_at='2018-08-01 23:59:59' and distribution_id in('','','') ORDER BY created_at;命中索引，但是有排序
EXPLAIN select * from tb_order WHERE pay_at='2018-08-01 23:59:59' and distribution_id in('','','');命中索引
EXPLAIN select * from tb_order WHERE distribution_id in('','','') and pay_at='2018-08-01 23:59:59' ORDER BY created_at;命中索引，但是有排序
EXPLAIN select * from tb_order WHERE distribution_id in('','','') and pay_at='2018-08-01 23:59:59';命中索引
EXPLAIN select * from tb_order WHERE pay_at='2018-08-01 23:59:59';没有命中索引

总结：

1、组合索引最左原则

2、查询出来的数据量小于总数据的30%，会走索引。

原理支撑：

Mysql索引原理

MySQL的InnoDB索引原理详解