我如何才能最好地编写一个查询,从总共600k中随机选择10行?


当前回答

这非常快,而且是100%随机的,即使你有间隙。

将SELECT Count(*)中可用的行数计算为rows FROM TABLE 选择10个不同的随机数a_1,a_2,…,a_10在0到x之间 SELECT * FROM TABLE LIMIT 1 offset a_i for i=1,…,10

我在Bill Karwin的《SQL反模式》一书中发现了这个破解方法。

其他回答

从书中:

使用偏移量选择随机行

这是另一种避免前面提到的问题的技术 替代方法是统计数据集中的行数并返回一个随机值 0到计数之间的数字。然后用这个数字作为抵消 查询数据集时

$rand = "SELECT ROUND(RAND() * (SELECT COUNT(*) FROM Bugs))";
$offset = $pdo->query($rand)->fetch(PDO::FETCH_ASSOC);
$sql = "SELECT * FROM Bugs LIMIT 1 OFFSET :offset";
$stmt = $pdo->prepare($sql);
$stmt->execute( $offset );
$rand_bug = $stmt->fetch();

在不能假定连续键值和时使用此解决方案 您需要确保每一行都有均等的机会被选中。

SELECT
  * 
FROM
  table_with_600k_rows
WHERE
  RAND( ) 
ORDER BY
  id DESC 
LIMIT 30;

Id是主键,按Id排序, 解释table_with_600k_rows,发现该行不扫描整个表

所有最好的答案都已经贴出来了(主要是那些引用了http://jan.kneschke.de/projects/mysql/order-by-rand/的链接)。

I want to pinpoint another speed-up possibility - caching. Think of why you need to get random rows. Probably you want display some random post or random ad on a website. If you are getting 100 req/s, is it really needed that each visitor gets random rows? Usually it is completely fine to cache these X random rows for 1 second (or even 10 seconds). It doesn't matter if 100 unique visitors in the same 1 second get the same random posts, because the next second another 100 visitors will get different set of posts.

当使用这种缓存时,你也可以使用一些较慢的解决方案来获取随机数据,因为不管你的req/s如何,它每秒只会从MySQL中获取一次。

另一个简单的解决方案是对行进行排名,并随机获取其中之一,有了这个解决方案,你将不需要在表中有任何基于“Id”的列。

SELECT d.* FROM (
SELECT  t.*,  @rownum := @rownum + 1 AS rank
FROM mytable AS t,
    (SELECT @rownum := 0) AS r,
    (SELECT @cnt := (SELECT RAND() * (SELECT COUNT(*) FROM mytable))) AS n
) d WHERE rank >= @cnt LIMIT 10;

您可以根据需要更改限制值,以便访问尽可能多的行,但大多数情况下是连续的值。

然而,如果你不想要连续的随机值,那么你可以获取一个更大的样本并从中随机选择。就像……

SELECT * FROM (
SELECT d.* FROM (
    SELECT  c.*,  @rownum := @rownum + 1 AS rank
    FROM buildbrain.`commits` AS c,
        (SELECT @rownum := 0) AS r,
        (SELECT @cnt := (SELECT RAND() * (SELECT COUNT(*) FROM buildbrain.`commits`))) AS rnd
) d 
WHERE rank >= @cnt LIMIT 10000 
) t ORDER BY RAND() LIMIT 10;

一个伟大的职位处理几个情况,从简单,到差距,到不均匀与差距。

http://jan.kneschke.de/projects/mysql/order-by-rand/

对于大多数一般情况,你可以这样做:

SELECT name
  FROM random AS r1 JOIN
       (SELECT CEIL(RAND() *
                     (SELECT MAX(id)
                        FROM random)) AS id)
        AS r2
 WHERE r1.id >= r2.id
 ORDER BY r1.id ASC
 LIMIT 1

这假设id的分布是相等的,并且id列表中可能存在间隙。有关更高级的示例,请参阅本文