联合索引 where 查询的不同情况
暂时记录一下, 有时间回来更新 , 这里涉及到了 解释器的优化
select count(id) from user
-- 查看索引
show index from user
-- 删除之前的索引
drop index idx_sex_pass on user
-- 创建联合索引 执行时间 42.493s
create index `idx_sex_email_nickname` on user(sex,email,nickname)
-- 测试 条件查询 字段顺序 -0.024s type ref
select username, `password` from user where sex = 'male' and email = 'jack1003@itcast.cn' and nickname = '1003'
-- 索引生效, 数据量大, 全表扫描 const
select username, `password` from user where sex = 'male'
-- 走索引 违背最左原则 6.229s
select username, `password` from user where nickname = '1003' and sex = 'male' and email = 'jack1003@itcast.cn'
-- 没有第一个字段, 不走索引 全表扫描 type All
select username, `password` from user where email = 'jack1003@itcast.cn' and nickname = '1003'
-- type ref 索引没有完全生效
select username, `password` from user where sex = 'male' and nickname = '1003'
-- type ref 索引没有完全生效
select username, `password` from user where email = 'jack1003@itcast.cn' and nickname = '1003' and sex = 'male'
-- type range
select username, `password` from user where nickname >'1003' and sex = 'male' and email = 'jack1003@itcast.cn'
select username, `password` from user where nickname = '10080' and sex = 'male' and email = 'jack10080@itcast.cn'
-- 没有第一个字段 不走索引
select username, `password` from user where nickname = '10080'
-- type ref
select username, `password` from user where nickname = '10080' and sex = 'male'