MySQL中随机select记录

我有这个需求:

数据库中的uid离散分布不连续,需要随机select某一条记录。

1、最懒做法

select uid, uname from user order by RAND() limit 1

这个非常慢,因为几乎要遍历整个表。

2、用id随机范围。

其实如果我们能得到min(uid)和max(uid),然后随机这之间的某一个ID,再where >= 就可以了。

首先是获取min和max的uid:

select min(uid), max(uid) from user;

然后是获取[min, max]之间的随机数。

select min(uid)+floor(RAND()*(max(uid)-min(uid))) from user;

上面这句执行非常慢,

+----+-------------+-------+-------+---------------+---------+---------+------+--------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows   | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+--------+-------------+
|  1 | SIMPLE      | user  | index | NULL          | PRIMARY | 4       | NULL | 921162 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+--------+-------------+
1 row in set (0.00 sec)

需要优化:

select ceil(rand()*(select max(uid) from user));

然后加上where

 select uid, uname from user as r1 join (select min(uid) as min, max(uid) as max from user) as r2 where r1.uid >= r2.min+floor((r2.max-r2.min)*RAND()) limit 0,1;

最后这句非常快了。

 

 

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