mysql 使用rand随机查询记录效率测试
一直以为mysql随机查询几条数据,就用
SELECT
*
FROM
`
table
`
ORDER
BY
RAND
() LIMIT
5
就可以了。
但是真正测试一下才发现这样效率非常低。一个15万余条的库,查询5条数据,居然要8秒以上
查看官方手册,也说rand()放在ORDER BY 子句中会被执行多次,自然效率及很低。
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,网上基本上都是查询max(id) * rand()来随机获取数据。
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 ;
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 ;
但是这样会产生连续的5条记录。解决办法只能是每次查询一条,查询5次。即便如此也值得,因为15万条的表,查询只需要0.01秒不到。
上面的语句采用的是JOIN,mysql的论坛上有人使用
SELECT
*
FROM ` table `
WHERE id >= ( SELECT FLOOR ( MAX (id) * RAND ()) FROM ` table ` )
ORDER BY id LIMIT 1 ;
FROM ` table `
WHERE id >= ( SELECT FLOOR ( MAX (id) * RAND ()) FROM ` table ` )
ORDER BY id LIMIT 1 ;
我测试了一下,需要0.5秒,速度也不错,但是跟上面的语句还是有很大差距。总觉有什么地方不正常。
于是我把语句改写了一下。
SELECT
*
FROM
`
table
`
WHERE id >= ( SELECT floor ( RAND () * ( SELECT MAX (id) FROM ` table `)))
ORDER BY id LIMIT 1 ;
WHERE id >= ( SELECT floor ( RAND () * ( SELECT MAX (id) FROM ` table `)))
ORDER BY id LIMIT 1 ;
这下,效率又提高了,查询时间只有0.01秒
最后,再把语句完善一下,加上MIN(id)的判断。我在最开始测试的时候,就是因为没有加上MIN(id)的判断,结果有一半的时间总是查询到表
中的前面几行。
完整查询语句是:
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 ;
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
看来采用JOIN的语法比直接在WHERE中使用函数效率还要高很多。