DISTINCT ON query w/ ORDER BY max value of a column

问题

I've been tasked with converting a Rails app from MySQL to Postgres asap and ran into a small issue.
The active record query:

current_user.profile_visits.limit(6).order("created_at DESC").where("created_at > ? AND visitor_id <> ?", 2.months.ago, current_user.id).distinct

Produces the SQL:

SELECT  visitor_id, MAX(created_at) as created_at, distinct on (visitor_id) *
FROM "profile_visits"
WHERE "profile_visits"."social_user_id" = 21
AND (created_at > '2015-02-01 17:17:01.826897' AND visitor_id <> 21)
ORDER BY created_at DESC, id DESC
LIMIT 6

I'm pretty confident when working with MySQL but I'm honestly new to Postgres. I think this query is failing for multiple reasons.

I believe the distinct on needs to be first.
I don't know how to order by the results of max function
Can I even use the max function like this?

The high level goal of this query is to return the 6 most recent profile views of a user. Any pointers on how to fix this ActiveRecord query (or it's resulting SQL) would be greatly appreciated.

回答1:

The high level goal of this query is to return the 6 most recent profile views of a user.

That would be simple. You don't need max() nor DISTINCT for this:

SELECT *
FROM   profile_visits
WHERE  social_user_id = 21
AND    created_at > (now() - interval '2 months')
AND    visitor_id <> 21  -- ??
ORDER  BY created_at DESC NULLS LAST, id DESC NULLS LAST
LIMIT  6;

I suspect your question is incomplete. If you want:
the 6 latest visitors with their latest visit to the page
then you need a subquery. You cannot get this sort order in one query level, neither with DISTINCT ON, nor with window functions:

SELECT *
FROM  (
   SELECT DISTINCT ON (visitor_id) *
   FROM   profile_visits
   WHERE  social_user_id = 21
   AND    created_at > (now() - interval '2 months')
   AND    visitor_id <> 21  -- ??
   ORDER  BY visitor_id, created_at DESC NULLS LAST, id DESC NULLS LAST
   ) sub
ORDER  BY created_at DESC NULLS LAST, id DESC NULLS LAST
LIMIT  6;

The subquery sub gets the latest visit per user (but not older than two months and not for a certain visitor21. ORDER BY must have the same leading columns as DISTINCT ON.

You need the outer query to get the 6 latest visitors then.
Consider the sequence of events: