mysql之 rand 随机查询记录效率
转载SELECT * FROM `table` ORDER BY RAND() LIMIT 5 不推荐
一个15万余条的库,查询5条数据,居然要8秒以上
官方手册:You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times.
搜索Google:
SELECT *
FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * (SELECT MAX(id) FROM `table`)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id ASC LIMIT 5;
产生连续的5条记录。解决办法只能是每次查询一条,查询5次;15万条的表,查询只需要0.01秒不到
MySQL的论坛:
SELECT *
FROM `table`
WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table` )
ORDER BY id LIMIT 1;
需要0.5秒,速度不错
改写:
SELECT * FROM `table`
WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `table`)))
ORDER BY id LIMIT 1;
效率提高,只有0.01秒
加上MIN(id)的判断:
SELECT * FROM `table`
WHERE id >= (SELECT floor( RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`)))
ORDER BY id LIMIT 1;
SELECT *
FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`))+(SELECT MIN(id) FROM `table`)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id LIMIT 1;
最后在php中对这两个语句进行分别查询10次,
前者花费时间 0.147433 秒
后者花费时间 0.015130 秒
下一篇:mysql limit查询优化
提问和评论都可以,用心的回复会被更多人看到
评论
发布评论
相关文章
-
MySQL优化查询的几种方式
MySQL优化查询的几种方式
索引 limit join -
mysql查询不重复的数据 mysql查询不重复的列
mysql-查询多列不重复的数据
mysql查询非重复数据结构 mysql 多列 数据 分组查询