Question

Trying to run this query to find the intersection of two sets (users who post on one page and users who post on another). Unfortunately, this takes ridiculous amounts of time to complete (10+ minutes). Is there any way to reduce its complexity?

SELECT DISTINCT (user_id) 
FROM facebook_post_comments 
WHERE page_id = some_page_id 
AND user_id IN ( SELECT DISTINCT(user_id) 
                 FROM facebook_post_comments 
                 WHERE page_id = some_other_page_id )

Answer 1

There’s a bug in MySQL where IN with a subquery sometimes gives very poor performance (fixed in MySQL 5.6).

Try this query using a JOIN instead:

SELECT DISTINCT user_id
FROM facebook_post_comments T1 
JOIN facebook_post_comments T2
    ON T1.user_id = T2.user_id
    AND T2.page_id = some_other_page_id
WHERE T1.page_id = some_page_id