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.

[scribu]

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.

[hakre]

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

[wonderboymusic]

A patch for this would be fun.

[MisdaX]

first attempt - passes all phpunit standard tests

[MisdaX]

Tests with MySQL EXPLAIN shows no creation of temporary tables. The execution time with ~900 posts is also 2-3 times faster.

I ran the standard phpunit tests - no additional error was found.

[johnbillion]

Looks like your patch got missed, MisdaX. Sorry about that.

Can we confirm which unit tests cover the ORDER BY RAND() functionality please? Not that it's really very testable, but still.

[MisdaX]

It is indeed difficult. I tried to reduce the impact of the patch as best as I can. I tested a vanilla wordpress and a patched wordpress with the whole unit test package to ensure that I broke nothing.

I assumed that there are tests which cover the ordering part of the function. No change in the result of the unit tests was a sign for me that everything should be ok. Unfortunately I have no idea how I can provide a suitable test for a random order functionality :-)

Independently, it would be nice if someone could provide a performane test with a big database.

This update modifies the 'test_orderby' function assertion to accommodate expected 'rand()' value in the SQL query request. An additional unit test 'test_wp_query_orderby_rand' is also introduced to verify that the 'orderby' => 'rand' parameter provides seemingly random results upon multiple requests.

Trac ticket: 18836

[pbearne]

Hi

I created a test to check we get a random order.

but the code provided in the patch did not provide a random order so more work is needed.

[pbearne]

may put into another ticket