Mysql随机取样——ORDER BY RAND()优化

The palest ink is better than best memory——好记性不如烂笔头。2013补记

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一、关键词：

随机取样、order by rand()

二、业务场景：

一款新产品上线后，刚开始用户比较少，不够活跃。但，出于运营需要，比如社交产品首页Feed流随机出老动态，系统修改发布时间显示~电商产品的商品列表随机显示商品，不至于每次用户看见的商品都一样——空城计——一种活跃的假象。所以研发就有了随机取样的活儿……

三、事例：

使用mysql的都知道，如下这样是随机取样的最直接方便的方式：

SELECT * FROM product_info  ORDER BY RAND() LIMIT 10

执行过程如下：

可是，ORDER BY RAND()超级慢，在200W记录表中，单次执行需要17s+….如果这样上线，用户并发访问，数据库肯定吃不消。

查阅MySQL官方手册得知：在ORDER BY从句里面不能使用RAND()函数，因为这样会导致数据列被多次扫描。
如下：http://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_rand

RAND() in a WHERE clause is evaluated for every row (when selecting from one table) or combination of rows (when selecting from a multiple-table join). Thus, for optimizer purposes, RAND() is not a constant value and cannot be used for index optimizations. For more information, see Section 9.2.1.3.5, “Function Call Optimization”.

Use of a column with RAND() values in an ORDER BY or GROUP BY clause may yield unexpected results because for either clause a RAND() expression can be evaluated multiple times for the same row, each time returning a different result.

搜索Google，网上基本上都是使用MAX() /MIN()、 RAND()、FLOOR()/ROUND()函数相互结合达到效果，如下SQL1：：

SELECT *
FROM `product_info` AS t1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(id) FROM `product_info`)-(SELECT MIN(id) FROM `product_info`))+(SELECT MIN(id) FROM `product_info`)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id LIMIT 10;

或简单一点如下SQL2：

SELECT * FROM `product_info` p join (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM `product_info`)) AS id ) as p1 
WHERE p.id >= p1.id
ORDER BY p.id LIMIT 10;

速度是挺快，但是结果集不够随机，往往数据在一个区间——不离散，所以不符合需求。

把SQL2改造如下SQL3（两者的区别就在于JOIN和FROM子查询的不同）：

SELECT * FROM `product_info`
WHERE id >= (SELECT FLOOR(RAND() * (SELECT MAX(id) FROM `product_info`))) 
ORDER BY id LIMIT 10;

查询一次，基本耗时在0.09s，执行过程如下：

把SQL3继续改成如下，结果较离散一些（执行效率差不多）：

SELECT * FROM `product_info`
WHERE id >= (SELECT FLOOR( RAND()*((SELECT MAX(id) FROM product_info)-(SELECT MIN(id) FROM product_info))+(SELECT MIN(id) FROM product_info)))
ORDER BY id LIMIT 10;