http://ourmysql.com/archives/143 我们致力于一个MySQL知识的分享网站 Sat, 12 May 2012 08:14:43 +0000 hourly 1 http://wordpress.org/?v=3.3.1 http://ourmysql.com/archives/143/comment-page-1#comment-46046 Jedy Tue, 01 Jun 2010 07:05:42 +0000 http://www.ourmysql.com/?p=143#comment-46046 join比where快是因为mysql做了多余的优化。 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; 实际上执行的是 SELECT * FROM `table` WHERE id >= floor( RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`)) ORDER BY id LIMIT 1; 变成了按id顺序读，每行判断id是否大于floor( RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`))，取第一次满足的行，所以用where并不是均匀随机的，得到的应该都是id比较小的行。当然这种方法本身无法保证均匀随机，所以join也不是，不过要好多了。 join比where快是因为mysql做了多余的优化。
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;
实际上执行的是
SELECT * FROM `table`
WHERE id >= floor( RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`))
ORDER BY id LIMIT 1;
变成了按id顺序读，每行判断id是否大于floor( RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`))，取第一次满足的行，所以用where并不是均匀随机的，得到的应该都是id比较小的行。当然这种方法本身无法保证均匀随机，所以join也不是，不过要好多了。

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