MySQL: Alternatives to ORDER BY RAND()

I've read about a few alternatives to MySQL's ORDER BY RAND() function, but most of the alternatives apply only to where on a single random result is needed.

Does anyone have any idea how to optimize a query that returns multiple random results, such as this:

   SELECT u.id, 
          p.photo 
     FROM users u, profiles p 
    WHERE p.memberid = u.id 
      AND p.photo != '' 
      AND (u.ownership=1 OR u.stamp=1) 
 ORDER BY RAND() 
    LIMIT 18 

UPDATE 2016

This solution works best using an indexed column.

Here is a simple example of and optimized query bench marked with 100,000 rows.

OPTIMIZED: 300ms

SELECT 
    g.*
FROM
    table g
        JOIN
    (SELECT 
        id
    FROM
        table
    WHERE
        RAND() < (SELECT 
                ((4 / COUNT(*)) * 10)
            FROM
                table)
    ORDER BY RAND()
    LIMIT 4) AS z ON z.id= g.id

note about limit ammount: limit 4 and 4/count(*). The 4s need to be the same number. Changing how many you return doesn't effect the speed that much. Benchmark at limit 4 and limit 1000 are the same. Limit 10,000 took it up to 600ms

note about join: Randomizing just the id is faster than randomizing a whole row. Since it has to copy the entire row into memory then randomize it. The join can be any table that is linked to the subquery Its to prevent tablescans.

note where clause: The where count limits down the ammount of results that are being randomized. It takes a percentage of the results and sorts them rather than the whole table.

note sub query: The if doing joins and extra where clause conditions you need to put them both in the subquery and the subsubquery. To have an accurate count and pull back correct data.

UNOPTIMIZED: 1200ms

SELECT 
    g.*
FROM
    table g
ORDER BY RAND()
LIMIT 4

PROS

4x faster than order by rand(). This solution can work with any table with a indexed column.

CONS

It is a bit complex with complex queries. Need to maintain 2 code bases in the subqueries

Here's an alternative, but it is still based on using RAND():

  SELECT u.id, 
         p.photo,
         ROUND(RAND() * x.m_id) 'rand_ind'
    FROM users u, 
         profiles p,
         (SELECT MAX(t.id) 'm_id'
            FROM USERS t) x
   WHERE p.memberid = u.id 
     AND p.photo != '' 
     AND (u.ownership=1 OR u.stamp=1) 
ORDER BY rand_ind
   LIMIT 18

This is slightly more complex, but gave a better distribution of random_ind values:

  SELECT u.id, 
         p.photo,
         FLOOR(1 + RAND() * x.m_id) 'rand_ind'
    FROM users u, 
         profiles p,
         (SELECT MAX(t.id) - 1 'm_id'
            FROM USERS t) x
   WHERE p.memberid = u.id 
     AND p.photo != '' 
     AND (u.ownership=1 OR u.stamp=1) 
ORDER BY rand_ind
   LIMIT 18

It is not the fastest, but faster then common ORDER BY RAND() way:

ORDER BY RAND() is not so slow, when you use it to find only indexed column. You can take all your ids in one query like this:

SELECT id
FROM testTable
ORDER BY RAND();

to get a sequence of random ids, and JOIN the result to another query with other SELECT or WHERE parameters:

SELECT t.*
FROM testTable t
JOIN
    (SELECT id
    FROM `testTable`
    ORDER BY RAND()) AS z ON z.id= t.id   
WHERE t.isVisible = 1
LIMIT 100; 

in your case it would be:

SELECT u.id, p.photo 
FROM users u, profiles p 
JOIN
    (SELECT id
    FROM users
    ORDER BY RAND()) AS z ON z.id = u.id   
WHERE p.memberid = u.id 
  AND p.photo != '' 
  AND (u.ownership=1 OR u.stamp=1) 
LIMIT 18 

It's very blunt method and it can be not proper with very big tables, but still it's faster than common RAND(). I got 20 times faster execution time searching 3000 random rows in almost 400000.

Order by rand() is very slow on large tables,

I found the following workaround in a php script:

Select min(id) as min, max(id) as max from table;

Then do random in php

$rand = rand($min, $max);

Then

'Select * from table where id>'.$rand.' limit 1';

Seems to be quite fast....

I ran into this today and was trying to use 'DISTINCT' along with JOINs, but was getting duplicates I assume because the RAND was making each JOINed row distinct. I muddled around a bit and found a solution that works, like this:

SELECT DISTINCT t.id, 
                t.photo 
       FROM (SELECT  u.id, 
                     p.photo,
                     RAND() as rand
                FROM users u, profiles p 
                 WHERE p.memberid = u.id 
                  AND p.photo != '' 
                  AND (u.ownership=1 OR u.stamp=1)
                ORDER BY rand) t
       LIMIT 18

Create a column or join to a select with random numbers (generated in for example php) and order by this column.

The solution I am using is also posted in the link below: https://stackoverflow.com/questions/1244555/how-can-i-optimize-mysqls-order-by-rand-function/25882872#25882872

I am assuming your users table is going to be larger than your profiles table, if not then it's 1 to 1 cardinality.

If so, I would first do a random selection on user table before joining with profile table.

First do selection:

SELECT *
FROM users
WHERE users.ownership = 1 OR users.stamp = 1

Then from this pool, pick out random rows through calculated probability. If your table has M rows and you want to pick out N random rows, the probability of random selection should be N/M. Hence:

SELECT *
FROM
(
    SELECT *
    FROM users
    WHERE users.ownership = 1 OR users.stamp = 1
) as U
WHERE 
    rand() <= $limitCount / (SELECT count(*) FROM users WHERE users.ownership = 1 OR users.stamp = 1)

Where N is $limitCount and M is the subquery that calculates the table row count. However, since we are working on probability, it is possible to have LESS than $limitCount of rows returned. Therefore we should multiply N by a factor to increase the random pool size.

i.e:

SELECT*
FROM
(
    SELECT *
    FROM users
    WHERE users.ownership = 1 OR users.stamp = 1
) as U
WHERE 
    rand() <= $limitCount * $factor / (SELECT count(*) FROM users WHERE users.ownership = 1 OR users.stamp = 1)

I usually set $factor = 2. You can set the factor to a lower value to further reduce the random pool size (e.g. 1.5).

At this point, we would have already limited a M size table down to roughly 2N size. From here we can do a JOIN then LIMIT.

SELECT * 
FROM
(
       SELECT *
        FROM
        (
            SELECT *
            FROM users
            WHERE users.ownership = 1 OR users.stamp = 1
        ) as U
        WHERE 
            rand() <= $limitCount * $factor / (SELECT count(*) FROM users WHERE users.ownership = 1 OR users.stamp = 1)
) as randUser
JOIN profiles
ON randUser.id = profiles.memberid AND profiles.photo != ''
LIMIT $limitCount

On a large table, this query will outperform a normal ORDER by RAND() query.

Hope this helps!

SELECT
	a.id,
	mod_question AS modQuestion,
	mod_answers AS modAnswers 
FROM
	b_ask_material AS a
	INNER JOIN ( SELECT id FROM b_ask_material WHERE industry = 2 ORDER BY RAND( ) LIMIT 100 ) AS b ON a.id = b.id