[wp-trac] [WordPress Trac] #18836: ORDER BY RAND() is slow

WordPress Trac wp-trac at lists.automattic.com
Sat Oct 1 16:59:04 UTC 2011

#18836: ORDER BY RAND() is slow
 Reporter:  scribu       |      Owner:
     Type:  enhancement  |     Status:  new
 Priority:  normal       |  Milestone:  Awaiting Review
Component:  Performance  |    Version:
 Severity:  minor        |   Keywords:
 WP_Query currently accepts 'orderby' => 'rand' which translates to ORDER

 This is very slow when you have many posts, since it effectively calls
 RAND() for each row.

 A faster way would be to call RAND() only once and put it in the LIMIT

 The only thing is that we have to make sure that the generated number is
 smaller than (total number of posts - number of posts to fetch).

 So, this would require to do an extra query to calculate the total. It
 should still be faster than the current method.

 If we want to get all the posts, we can get them in any order and then
 randomise their order using shuffle() in PHP.

Ticket URL: <http://core.trac.wordpress.org/ticket/18836>
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