如何在MySQL中有效地选择随机记录？

Question

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,我该怎么做呢？

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     | <derived2> | system | NULL          | NULL | NULL    | NULL |     1 |                              |
|  1 | PRIMARY     | <derived3> | ALL    | NULL          | NULL | NULL    | NULL |    10 |                              |
|  3 | DERIVED     | r          | ALL    | NULL          | NULL | NULL    | NULL | 62228 | Using where                  |
|  2 | DERIVED     | NULL       | NULL   | NULL          | NULL | NULL    | NULL |  NULL | Select tables optimized away |
+----+-------------+------------+--------+---------------+------+---------+------+-------+------------------------------+

Answer 1

Quassnoi写了一篇关于随机选择行而不执行排序的文章。他的示例随机选择 10 行，但您可以对其进行调整以仅选择一行。

如果您希望它非常快，那么您可以使用不完全一致或有时无法返回行的近似值。

您还可以使用存储过程从Bill Karwin 的帖子中快速选择随机行：

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

请注意，这在 MyISAM 中比 InnoDB 运行得快得多，因为 COUNT(*) 在 InnoDB 中很昂贵，但在 MyISAM 中几乎是即时的。