Optimize this SQL Query

Question

This SQL query disgusts me. I didn't write it, but it's a massive cause of issues on our servers. I'm willing to split it up into multiple queries and do some of the processing via PHP (like, the RAND()).

$sql = "SELECT a.code, a.ad_id, a.position, a.type, a.image, a.url, a.height, a.width
    FROM " . AD_TABLE ." a, " . USER_GROUP_TABLE . " g
    WHERE (a.max_views >= a.views OR a.max_views = '0')
    AND (FIND_IN_SET(" .$forum_id. ", a.show_forums) > 0 OR a.show_all_forums = '1')
    AND g.user_id = " . $user->data['user_id'] . "
    AND FIND_IN_SET(g.group_id, a.groups)
    AND FIND_IN_SET(" . $user->data['user_rank'] . ", a.ranks)
    AND a.start_time < " . time() . "
    AND a.end_time > " . time() . "
    AND (a.clicks <= a.max_clicks OR a.max_clicks = '0')
    ORDER BY rand()";

Yeesh, I feel icky after pasting that...

EDIT: Below is the results of the "EXPLAIN" on a sample query in the above format, comma delimited:

"id","select_type","table","type","possible_keys","key","key_len","ref","rows","Extra"
1,"SIMPLE","g","ref","user_id","user_id","3","const",6,"Using temporary; Using filesort"
1,"SIMPLE","a","ALL","max_views","","","",10,"Using where"

It is

Answer 1

You have three major issues here:

1. FIND_IN_SET.

   It's not sargable, an index cannot make it faster. Create a many-to-many relationship table (or tables).

   • a.start_time < GETDATE() AND a.end_time > GETDATE()

   MySQL is not good in optimizing that. You can keep you timespans as geometry boxes and create a SPATIAL INDEX over them, this will be much faster (though less readable)

   • ORDER BY RAND()

   If you are using this to sample data (i. e. you don't need all rows but rather a small random subset), there is a more efficient way to do this, described in this article in my blog:

   • Selecting random rows