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

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.

It depends on how random you need to be. The solution you linked works pretty well IMO. Unless you have large gaps in the ID field, it's still pretty random.

However, you should be able to do it in one query using this (for selecting a single value):

SELECT [fields] FROM [table] WHERE id >= FLOOR(RAND()*MAX(id)) LIMIT 1

Other solutions:

• Add a permanent float field called random to the table and fill it with random numbers. You can then generate a random number in PHP and do "SELECT ... WHERE rnd > $random"
• Grab the entire list of IDs and cache them in a text file. Read the file and pick a random ID from it.
• Cache the results of the query as HTML and keep it for a few hours.