标签：pre weight tab rom val 随机 mamicode des mysq
order by limit会出现数据重复问题
--查看mysql版本 select version() --创建表 create table student( id int(10) PRIMARY KEY auto_increment, name varchar(32), age int(3) ) --插入数据 insert into student VALUEs(null,‘小明‘,11); insert into student VALUEs(null,‘王小明‘,5); insert into student VALUEs(null,‘小二毛‘,11); insert into student VALUEs(null,‘王小三‘,4); insert into student VALUEs(null,‘毛小二‘,11); insert into student VALUEs(null,‘张小二‘,11); --插入表 select * from student s order by age desc --分页查询 select * from(select * from student s order by age desc LIMIT 0,1)tab union all select * from(select * from student s order by age desc LIMIT 1,1)tab union all select * from(select * from student s order by age desc LIMIT 2,1)tab UNION ALL select * from(select * from student s order by age desc LIMIT 3,1)tab
我是5.6版本无法测试，5.7及其以上版本会出现 order by的列有相同的值时 同时未做索引时， mysql会随机选取这些行 文档地址:https://dev.mysql.com/doc/refman/5.7/en/limit-optimization.html
解决方式 可以增加一列含索引的排序字段
--分页查询 select * from(select * from student s order by age desc,id asc LIMIT 0,1)tab union all select * from(select * from student s order by age desc,id asc LIMIT 1,1)tab union all select * from(select * from student s order by age desc,id asc LIMIT 2,1)tab UNION ALL select * from(select * from student s order by age desc,id asc LIMIT 3,1)tab
select * from tab limit n,m limit原理是先读取前n条数据，读取最后m条数据 所以n越大就越慢 性能也越差
select * from `cpn_coupon_code` c limit 10000000,10
SELECT * FROM cpn_coupon_code c INNER JOIN (SELECT id FROM cpn_coupon_code e LIMIT 10000000,10)tab on tab.id=c.id
与优化前版本是先查出数据id 然后根据id查数据
因为只查索引页 索引 n,m n就只会是索引 不会包含数据
标签：pre weight tab rom val 随机 mamicode des mysq
原文地址：https://www.cnblogs.com/LQBlog/p/12208536.html
