MySQL Poor Performance with Not Exists Query

Question

I have an app in production right now that makes a MySQL query to select columns in a table that do not have an associated row in another table To do so, I am currently using a MySQL exists query that appears to becoming slower and slower the more results that are added.

This is an example of the current query:

SELECT m.ig_media_id, m.media_promotion_id FROM media_promotions m
WHERE m.likes_received < m.likes_requested 
AND m.user_id != 19 
AND m.active = 1
AND NOT EXISTS 
    (SELECT * from media_actions m_a WHERE m_a.user_id = 19 AND m_a.ig_media_id = m.ig_media_id)
ORDER BY m.created_at ASC
LIMIT 1

What can I do to speed this up? It is currently running at ~3 seconds and really slowing down my application.

Answer 1

This is the subquery:

NOT EXISTS (SELECT *
            from media_actions m_a
            WHERE m.user_id = 19 AND m_a.ig_media_id = m.ig_media_id
           )

You can speed this up with an index:

create index idx_media_actions on media_actions(ig_media_id, user_id)

You can also phrase this as:

NOT EXISTS (SELECT 1
            from media_actions m_a
            WHERE m.user_id = 19 AND m_a.ig_media_id = m.ig_media_id
           )

However, I think MySQL is smart enough to ignore the * in such a subquery.