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

[WordPress Trac]

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

 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
 clause.

 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.

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