MySQL: Alternatives to ORDER BY RAND()

Question

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.

Answer 1

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.