ORDER BY RAND() is slow

[scribu]

WP_Query currently accepts 'orderby' => 'rand' which translates to ORDER BY RAND().

This is very slow when you have many posts, since it effectively calls RAND() for each row.

A faster way would be to call RAND() only once and put it in the LIMIT clause.

The only thing is that we have to make sure that the generated number is smaller than (total number of posts - number of posts to fetch).

So, this would require to do an extra query to calculate the total. It should still be faster than the current method.

If we want to get more than one post, we can get them in any order and then call shuffle() on the resulting array.

Here's a method of doing it all in a single query:

​http://explainextended.com/2009/03/01/selecting-random-rows/

Not sure how feasible it would be for WP.

​http://stackoverflow.com/questions/1823306/mysql-alternatives-to-order-by-rand