How can i optimize MySQL's ORDER BY RAND() function?

Question

I'd like to optimize my queries so I look into mysql-slow.log.

Most of my slow queries contains ORDER BY RAND(). I cannot find a real solution to resolve this problem. Theres is a possible solution at MySQLPerformanceBlog but I don't think this is enough. On poorly optimized (or frequently updated, user managed) tables it doesn't work or I need to run two or more queries before I can select my PHP-generated random row.

Is there any solution for this issue?

A dummy example:

SELECT  accomodation.ac_id,         accomodation.ac_status,         accomodation.ac_name,         accomodation.ac_status,         accomodation.ac_images FROM    accomodation, accomodation_category WHERE   accomodation.ac_status != 'draft'         AND accomodation.ac_category = accomodation_category.acat_id         AND accomodation_category.acat_slug != 'vendeglatohely'         AND ac_images != 'b:0;' ORDER BY         RAND() LIMIT 1 

Answer 1

Try this:

SELECT  * FROM    (         SELECT  @cnt := COUNT(*) + 1,                 @lim := 10         FROM    t_random         ) vars STRAIGHT_JOIN         (         SELECT  r.*,                 @lim := @lim - 1         FROM    t_random r         WHERE   (@cnt := @cnt - 1)                 AND RAND(20090301) < @lim / @cnt         ) i 

This is especially efficient on MyISAM (since the COUNT(*) is instant), but even in InnoDB it's 10 times more efficient than ORDER BY RAND().

The main idea here is that we don't sort, but instead keep two variables and calculate the running probability of a row to be selected on the current step.

See this article in my blog for more detail:

• Selecting random rows

Update:

If you need to select but a single random record, try this:

SELECT  aco.* FROM    (         SELECT  minid + FLOOR((maxid - minid) * RAND()) AS randid         FROM    (                 SELECT  MAX(ac_id) AS maxid, MIN(ac_id) AS minid                 FROM    accomodation                 ) q         ) q2 JOIN    accomodation aco ON      aco.ac_id =         COALESCE         (         (         SELECT  accomodation.ac_id         FROM    accomodation         WHERE   ac_id > randid                 AND ac_status != 'draft'                 AND ac_images != 'b:0;'                 AND NOT EXISTS                 (                 SELECT  NULL                 FROM    accomodation_category                 WHERE   acat_id = ac_category                         AND acat_slug = 'vendeglatohely'                 )         ORDER BY                 ac_id         LIMIT   1         ),         (         SELECT  accomodation.ac_id         FROM    accomodation         WHERE   ac_status != 'draft'                 AND ac_images != 'b:0;'                 AND NOT EXISTS                 (                 SELECT  NULL                 FROM    accomodation_category                 WHERE   acat_id = ac_category                         AND acat_slug = 'vendeglatohely'                 )         ORDER BY                 ac_id         LIMIT   1         )         ) 

This assumes your ac_id's are distributed more or less evenly.