ORDER BY alternative values from main- and subquery - sql

Question

Right now I have a query that looks something like so:
SELECT id
FROM post_table post
WHERE post.post_user_id = user_id
ORDER BY (SELECT max(comment_created_date)
FROM comments_table WHERE comments_post_id = post.id) DESC,
post.post_created_date DESC
My idea for this query was that it would order a series of posts like so
Post 1:
created Date: Jan 1 2015
comments : []
Post 2:
created Date: Jan 5 2015
comments : []
Post 3:
created Date : December 1 2014
comments: [
0: Created Date: Jan 6 2015
]
So in this case the order the posts would be returned in is
Post 3, Post 2, Post 1
Because Post 3 has a comment that is newer than any other posts, and Post 2 was created before Post 1.
But when I run the query, the posts are still all sorted by their created date and the query doesn't seem to take into account the comments created date.

Answer 1

It should work like this:
SELECT id
FROM post_table p
WHERE post_user_id = $user_id -- this is your input parameter
ORDER BY GREATEST(
(
SELECT max(comment_created_date)
FROM comments_table
WHERE comments_post_id = p.id
)
, post_created_date) DESC NULLS LAST;
You will want to add NULLS LAST if date columns can be NULL.
PostgreSQL sort by datetime asc, null first?
If comments can only be later than posts (would make sense), you can use COALESCE instead of GREATEST.
Cleaner alternative (may or may not be faster, depending on data distribution):
SELECT id
FROM post_table p
LEFT JOIN (
SELECT comments_post_id AS id, max(comment_created_date) AS max_date
FROM comments_table
GROUP BY 1
) c USING (id)
WHERE post_user_id = $user_id
ORDER BY GREATEST(c.max_date, p.post_created_date) DESC NULLS LAST;
Since you have pg 9.3 you can also use a LATERAL join. Probably faster:
SELECT id
FROM post_table p
LEFT JOIN LATERAL (
SELECT max(comment_created_date) AS max_date
FROM comments_table
WHERE comments_post_id = p.id
GROUP BY comments_post_id
) c ON TRUE
WHERE post_user_id = $user_id
ORDER BY GREATEST(c.max_date, p.post_created_date) DESC NULLS LAST;

Answer 2

The other thing I would suggest, if this is going to be a heavily hit query, is to create a calculated column on the post_table of 'date_last_commented'
and use a trigger to update that on any insert or update of the comments table.
Then you could run simple query of:
SELECT id
FROM post_table post
WHERE post.post_user_id = user_id
ORDER BY post.date_last_commented DESC nulls last, post.post_created_date DESC
;