Question

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How do I select random rows in MySQL?

Okay, so I have a table storing game data, and it’s reaching 7 million rows.

I need an efficient way to select a random row.

Here’s what I’ve tried so far:

SELECT `id` FROM `information` ORDER BY RAND() LIMIT 1;

Fairly logical query. Takes 8 seconds to run. Completely unacceptable.

Second attempt:

SELECT FLOOR(RAND()*COUNT(`id`)) INTO @target FROM `information`;
SELECT `id` FROM `information` LIMIT @target,1;

Fails on syntax error with the variable in the second query.

Third attempt:

SELECT `id` FROM (SELECT `id` FROM `information`) `a` ORDER BY RAND() LIMIT 1

The idea here is to strip out all the other fields, since they’re unnecessary bulk. It half-works, bringing the time down to 6 seconds. Still far too long, though.

And… I’m out. I can’t think of any other ways (but it is 1 AM…), so any suggestions would be much appreciated. Thanks!

Answer 1

If you are using INNODB engine, an article I read before suggested that it is better to count the number of records first using COUNT(*) because the engine does not cache the count of the table unlike MyISAM,

DECLARE @count INT;
DECLARE @offset INT;

SET @count = (SELECT COUNT(*) FROM `information`);
SET @offset = CONVERT(FLOOR(RAND() * @count), SIGNED);

PREPARE mystatement FROM "SELECT `id` FROM `information` ORDER BY RAND() LIMIT ?, 1";
EXECUTE mystatement USING @offset;
DEALLOCATE PREPARE mystatement;

• SQLFiddle Demo

DISCLAIMER: the query i used performed exactly as needed on 3m records. I don’t know how it behaves on 7m records though.