How to efficiently select a random entry in MySQL?

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 |
+----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+

The above does not qualify as effective, how should I do it right?

UPDATE

Using the solution mentioned in the answer does not seem to help anyway:

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 wrote a message about selecting strings at random without performing sorting. His example selects 10 rows at random, but you can adapt it to select only one row.

If you want it to be really fast, you can use an approximation that will not be completely uniform or sometimes will not be able to return a string.

Bill Karwin:

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

, MyISAM, InnoDB, COUNT (*) InnoDB, MyISAM.

Answer 2

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