Fetching RAND() rows without ORDER BY RAND() in just one query

问题

Using RAND() in MySQL to get a single random row out of a huge table is very slow:

SELECT quote FROM quotes ORDER BY RAND() LIMIT 1

Here is an article about this issue and why this is the case.

Their solution is to use two queries:

SELECT COUNT(*) AS cnt FROM quotes

- Use result to generate a number between 0 and COUNT(*)

SELECT quote FROM quotes LIMIT $generated_number, 1

I was wondering, whether this would be possible in just one query.

So my approach was:

SELECT * FROM quotes
LIMIT (
  ROUND(
    (SELECT COUNT(*) FROM quotes) * RAND()
  )
), 1

But it seams MySQL does not allow any logic within Limit. Though I can not find any information about this topic, whether this is true.

So my Questions:

How can I use RAND() within LIMIT?
Do you know of any other way to solve this with just one query?

回答1:

Is there a reason why a stored procedure cannot be used to create a prepared statement?

DELIMITER //
DROP PROCEDURE IF EXISTS rand_quote//
CREATE PROCEDURE rand_quote()
BEGIN
    SET @rand := ROUND((SELECT COUNT(*) FROM quotes) * RAND());
    SET @sql := CONCAT('SELECT * FROM quotes LIMIT ', @rand, ', 1');
    PREPARE stmt FROM @sql;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;
END;
//
DELIMITER ;

回答2:

I just figgured this one, that seams like a solution:

SELECT * FROM quotes
WHERE quotes_id = ROUND(
  (SELECT COUNT(*) FROM quotes) * RAND()
)
LIMIT 1

But it will work only if quotes_id has no gaps.

回答3:

I solved the problem by checking max id. Then i made php loop of rand(0, max_id) which checks if object exists. Done.

Much faster then previous ordering by rand.

来源：https://stackoverflow.com/questions/3311039/fetching-rand-rows-without-order-by-rand-in-just-one-query

标签

mysql

random

query-optimization