random select query

SELECT  *
FROM    (
         SELECT  @cnt := COUNT(*) + 1,
                 @lim := 250
         FROM    chatlogs
        ) vars
STRAIGHT_JOIN
        (
         SELECT  r.*,
         @lim := @lim - 1
         FROM    chatlogs r
         WHERE   (@cnt := @cnt - 1)
                 AND RAND() < @lim / @cnt
                 AND channel = ?
                 AND username = ?
                 AND LENGTH(message) < 200
         ) i\