How do I select a random record efficiently in MySQL?

问题

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 doesn't qualify as efficient,how should I do it properly?

UPDATE

Seems using the solution mentioned in the answer still doesn't help:

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

回答1:

Quassnoi has written a post about selecting rows at random without performing a sort. His example selects 10 rows at random, but you can adapt it to select just one row.

If you want it to be really fast then you can use an approximation that won't be completely uniform or will sometimes fail to return a row.

You can also use a stored procedure to select a random row quickly from Bill Karwin's post:

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

Note that this will run much faster in MyISAM than InnoDB because COUNT(*) is expensive in InnoDB but nearly instant in MyISAM.

回答2:

Well, If you can move some logic to application layer (and I didn't misunderstood your question), then all you need is to generate random ID in your application and then perform simple select for one record identified by that key. All you need to know is count of records. Oh, and if that key was deleted, get next one.

来源：https://stackoverflow.com/questions/2707717/how-do-i-select-a-random-record-efficiently-in-mysql

标签

mysql

sql-optimization