Re: Slow index scan on B-Tree index over timestamp field

From: pgsql-performance-owner@postgresql.org [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of Caio Casimiro
Sent: Monday, November 04, 2013 3:44 PM
To: Jeff Janes
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] Slow index scan on B-Tree index over timestamp field

Thank you very much for your answers guys!

On Mon, Nov 4, 2013 at 5:15 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
On Sun, Nov 3, 2013 at 2:05 PM, Caio Casimiro <casimiro.listas@gmail.com> wrote:
Hello all,

I have one query running at ~ 7 seconds and I would like to know if it's possible to make it run faster, once this query runs lots of time in my experiment.


Do you mean you want it to be fast because it runs many times, or that you want it to become fast after it runs many times (i.e. once the data is fully cached)?  The plan you show takes 24 seconds, not 7 seconds.

I want it to be fast because it runs many times. I have an experiment that evaluates recommendation algorithms  for a set of twitter users. This query returns recommendation candidates so it is called a lot of times for different users and time intervals.
 
 

Basically the query return the topics of tweets published by users that the user N follows and that are published between D1 and D2.

Query:

SELECT tt.tweet_id, tt.topic, tt.topic_value
            FROM tweet_topic AS tt LEFT JOIN tweet AS t ON tt.tweet_id = t.id
            WHERE creation_time BETWEEN 'D1' AND 'D2' AND user_id in
            (SELECT followed_id FROM relationship WHERE follower_id = N) ORDER BY tt.tweet_id;


I don't know if this affects the plan at all, but it is silly to do a left join to "tweet" when the WHERE clause has conditions that can't be satisfied with a null row.  Also, you could try changing the IN-list to an EXISTS subquery.

I'm sorry the ignorance, but I don't understand the issue with the left join, could you explain more?

...........................................

Thank you very much again!
Caio

Just try the following:

SELECT tt.tweet_id, tt.topic, tt.topic_value
            FROM tweet_topic AS tt  JOIN tweet AS t ON (tt.tweet_id = t.id
                                                  AND t.creation_time BETWEEN 'D1' AND 'D2' AND t.user_id in
                                         (SELECT followed_id FROM relationship WHERE follower_id = N))
 ORDER BY tt.tweet_id;

And see if it helps with performance.

Regards,
Igor Neyman