MySQL JOIN + Subquery Query Optimization

Question

I'm trying to fetch 100 posts and order them by the number of times they've been "remixed" in the last week. Here is my query thus far:

SELECT COUNT(remixes.post_id) AS count, posts.title
FROM posts 
LEFT JOIN (
    SELECT * FROM remixes WHERE created_at >= 1343053513
) AS remixes ON posts.id = remixes.post_id
GROUP BY posts.id 
ORDER BY count DESC, posts.created_at DESC
LIMIT 100

This produces the correct result; however, after running DESCRIBE I get this:

And here are my indexes on posts :

And my indexes on remixes :

And here are my questions:

1. Can you explain what the terms used in the extra column are really trying to tell me?
2. Could you provide tips on how I can optimize this query so that it'll scale better.

Thanks in advance!

Update

Per Zane's solution, I've updated my query to:

SELECT COUNT(remixes.post_id) AS count, posts.title
FROM posts 
LEFT JOIN remixes ON posts.id = remixes.post_id AND remixes.created_at >= 1343053513
GROUP BY posts.id 
ORDER BY count DESC, posts.created_at DESC
LIMIT 100

And here's the latest DESCRIBE

I'm still worried about the filesort part. Any ideas?

Answer 1

Try not to wrap your JOIN in a sub-select as this will create an unindexed temporary table to store the result of the subselect in, where it then joins on that unindexed table.

Instead, put created_at as an additional join condition when joining the remixes table:

SELECT 
    a.title, COUNT(b.post_id) AS remixcnt
FROM 
    posts a
LEFT JOIN 
    remixes b ON a.id = b.post_id AND b.created_at >= 1343053513
GROUP BY 
    a.id, a.title
ORDER BY 
    remixcnt DESC, a.created_at DESC
LIMIT 100

Answer 2

It seems to me that

SELECT COUNT(remixes.post_id) AS count, posts.title
FROM posts 
LEFT JOIN (
    SELECT * FROM remixes WHERE created_at >= 1343053513
) AS remixes ON posts.id = remixes.post_id
GROUP BY posts.id 
ORDER BY count DESC, posts.created_at DESC
LIMIT 100

could be rewritten as

SELECT COUNT(r.post_id) AS count, posts.title
FROM posts 
LEFT JOIN remixes r ON posts.id = r.post_id
WHERE r.created_at >= 1343053513
GROUP BY posts.id 
ORDER BY count DESC, posts.created_at DESC
LIMIT 100

which should give you a better EXPLAIN plan and run faster.