如何在MySQL中有效地选择随机记录？

MysqL> EXPLAIN SELECT * FROM urls ORDER BY RAND() liMIT 1;+----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+| ID | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra                           |+----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+|  1 | SIMPLE      | urls  | ALL  | NulL          | NulL | NulL    | NulL | 62228 | Using temporary; Using filesort |+----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+

以上不符合效率,我该如何正确地做到？

UPDATE

似乎使用答案中提到的解决方案仍无济于事：

MysqL> explain SELECT  *    -> FROM    (    ->         SELECT  @cnt := COUNT(*) + 1,->                 @lim := 10    ->         FROM    urls    ->         ) vars    -> STRAIGHT_JOIN    ->         (    ->         SELECT  r.*,->                 @lim := @lim - 1    ->         FROM    urls r    ->         WHERE   (@cnt := @cnt - 1)    ->                 AND RAND(20090301) < @lim / @cnt    ->         ) i;+----+-------------+------------+--------+---------------+------+---------+------+-------+------------------------------+| ID | select_type | table      | type   | possible_keys | key  | key_len | ref  | rows  | Extra                        |+----+-------------+------------+--------+---------------+------+---------+------+-------+------------------------------+|  1 | PRIMARY     | 

SET @r := (SELECT ROUND(RAND() * (SELECT COUNT(*) FROM mytable)));SET @sql := CONCAT('SELECT * FROM mytable liMIT ',@r,',1');PREPARE stmt1 FROM @sql;EXECUTE stmt1;