MYSQL Query very slow when using ORDER BY datetime - php

Question

I have the following query which runs on a social network. The query fetches posts (like Facebook posts) from a database.
SELECT P.*,
P.id_post id_p,
PM.meta_content video_title,
PM2.meta_content video_views,
PM3.meta_content racebooking_views,
Greatest(P.creation_date, Coalesce(Max(C.date), P.creation_date)) AS
last_activity,
P.creation_date creation_date,
(SELECT Count(*)
FROM likes
WHERE post_id = P.id_post
AND post_type = 'P')
likes_count,
(SELECT Count(*)
FROM likes L
WHERE post_id = P.id_post
AND post_type = 'P'
AND L.id_profile = 2796)
do_i_like
FROM posts P
LEFT JOIN comments C
ON P.id_post = C.post_id
AND C.post_type = 'P'
AND C.id_profile != P.id_profile
LEFT JOIN post_meta PM
ON PM.id_post = P.id_post
AND PM.meta_type = 'T'
LEFT JOIN post_meta PM2
ON PM2.id_post = P.id_post
AND PM2.meta_type = 'V'
LEFT JOIN post_meta PM3
ON PM3.id_post = P.id_post
AND PM3.meta_type = 'W'
GROUP BY P.id_post
ORDER BY last_activity DESC
LIMIT 41, 10
Each post may have or may not have comments.
I want the query to fetch the post with the most recent activity first.
So, if the post has a comment, i take the date of the latest comment. If the post does not have a comment, i take the creation date of the post.
The job is done by Greatest(P.creation_date, Coalesce(Max(C.date), P.creation_date)) which picks up the greates value between the comments dates (if comments exist) and the post creation date.
Then, the ORDER BY last_activity DESC does the sorting job.
PROBLEM
The query is really slow. It takes 8 seconds to run. The posts table has 8K rows and the comments table has 8K rows.
What i don't understand is that if I replace the ORDER BY clause with this ORDER BY P.id_post it takes 0.5 seconds to run. But if I replace the ORDER BY clause with ORDER BY P.creation_date again it takes 8 seconds. It seems that it doesn't like dates...
Additional infos
posts table has an index on creation_date.
comments table has an index on date
server runs LAMP on CentOS Linux 6.6
I tried other solutions on SO like this one but they didn't work
How can i fix this query to run faster?

Answer 1

The correlated subqueries in the select clause are probably killing you. Instead, join to a subquery which computes likes statistics:
SELECT P.*,
P.id_post id_p,
PM.meta_content video_title,
PM2.meta_content video_views,
PM3.meta_content racebooking_views,
GREATEST(P.creation_date, COALESCE(MAX(C.date), P.creation_date)) AS last_activity,
P.creation_date creation_date,
t.likes_count,
t.do_i_like
FROM posts P
LEFT JOIN
(
SELECT
post_id,
SUM(CASE WHEN post_type = 'P' THEN 1 ELSE 0 END) AS likes_count,
SUM(CASE WHEN post_type = 'P' AND L.id_profile = 2796
THEN 1 ELSE 0 END) AS do_i_like
FROM likes
GROUP BY post_id
) t
ON t.post_id = P.id_post
LEFT JOIN comments C
ON P.id_post = C.post_id AND
C.post_type = 'P' AND
C.id_profile != P.id_profile
LEFT JOIN post_meta PM
ON PM.id_post = P.id_post AND
PM.meta_type = 'T'
LEFT JOIN post_meta PM2
ON PM2.id_post = P.id_post AND
PM2.meta_type = 'V'
LEFT JOIN post_meta PM3
ON PM3.id_post = P.id_post AND
PM3.meta_type = 'W'
ORDER BY
last_activity DESC
LIMIT 41, 10
Also after editing your query I do not see a reason to be using GROUP BY in the outer query, so I removed it. And you should be using indices where appropriate, though my hunch is that my suggestion alone should give a noticeable performance boost.

Answer 2

There is a MAX(C.Date) that would require a group by clause, however it too could be substituted for a subquery I believe:
SELECT P.*,
P.id_post id_p,
PM.meta_content video_title,
PM2.meta_content video_views,
PM3.meta_content racebooking_views,
GREATEST(P.creation_date, COALESCE(max_c_date, P.creation_date)) AS last_activity,
P.creation_date creation_date,
t.likes_count,
t.do_i_like
FROM posts P
LEFT JOIN
(
SELECT
post_id,
SUM(CASE WHEN post_type = 'P' THEN 1 ELSE 0 END) AS likes_count,
SUM(CASE WHEN post_type = 'P' AND L.id_profile = 2796
THEN 1 ELSE 0 END) AS do_i_like
FROM likes
GROUP BY post_id
) t
ON t.post_id = P.id_post
LEFT JOIN (
SELECT
comments.post_id,
MAX(comments.date) max_c_date
FROM comments
inner join posts ON comments.post_id = posts.id_post
where comments.post_type = 'P' AND
comments.id_profile != posts.id_profile
GROUP BY comments.post_id
) C
ON P.id_post = C.post_id AND
LEFT JOIN post_meta PM
ON PM.id_post = P.id_post AND
PM.meta_type = 'T'
LEFT JOIN post_meta PM2
ON PM2.id_post = P.id_post AND
PM2.meta_type = 'V'
LEFT JOIN post_meta PM3
ON PM3.id_post = P.id_post AND
PM3.meta_type = 'W'
ORDER BY
last_activity DESC
LIMIT 41, 10