Re: From Simple to Complex - Mailing list pgsql-performance
| From | Bob Lunney |
|---|---|
| Subject | Re: From Simple to Complex |
| Date | |
| Msg-id | 1328123044.31389.YahooMailNeo@web39707.mail.mud.yahoo.com Whole thread Raw |
| In response to | Re: From Simple to Complex (Alessandro Gagliardi <alessandro@path.com>) |
| Responses |
Re: From Simple to Complex
(Alessandro Gagliardi <alessandro@path.com>)
|
| List | pgsql-performance |
Possibly. What does
psql > show work_mem;
say?
Bob Lunney
From: Alessandro Gagliardi <alessandro@path.com>
To: pgsql-performance@postgresql.org
Sent: Wednesday, February 1, 2012 12:19 PM
Subject: Re: [PERFORM] From Simple to Complex
Final update on this thread: since it is only necessary for me to get a rough ratio of the distribution (and not the absolute count), I refactored the query to include a subquery that samples from the moments table thus: SELECT moment_id, block_id FROM moments WHERE inserted BETWEEN 'yesterday' AND 'today' ORDER BY RANDOM() LIMIT 10000; I also took advantage of another table called blocks that happens to contain the moment_type as well (thus making it so I don't need to reference pg_class). The final query looks like:
SELECT moment_type, emotion, COUNT(feedback_id)
FROM (SELECT moment_id, block_id
FROM moments
WHERE inserted BETWEEN 'yesterday' AND 'today'
ORDER BY RANDOM() LIMIT 10000) AS sample_moments
JOIN blocks USING (block_id)
JOIN emotions USING (moment_id)
GROUP BY moment_type, emotion
ORDER BY moment_type, emotion
The explain is at http://explain.depesz.com/s/lYh
Interestingly, increasing the limit does not seem to increase the runtime in a linear fashion. When I run it with a limit of 60000 I get a runtime of 14991 ms. But if I run it with a limit of 70000 I get a runtime of 77744 ms. I assume that that's because I'm hitting a memory limit and paging out. Is that right?
On Tue, Jan 31, 2012 at 3:43 PM, Alessandro Gagliardi <alessandro@path.com> wrote:
I just got a pointer on presenting EXPLAIN ANALYZE in a more human friendly fashion (thanks, Agent M!): http://explain.depesz.com/s/A9SFrom this it looks like the bottleneck happens when Postgres does an Index Scan using emotions_moment_id_idx on emotions before filtering on moments.inserted so I thought I'd try filtering on emotions.inserted instead but that only made it worse. At the same time, I noticed that "FROM pg_class, moments WHERE moments.tableoid = pg_class.oid" tends to run a bit faster than "FROM pg_class JOIN moments ON moments.tableoid = pg_class.oid". So I tried:SELECT relname, emotion, COUNT(feedback_id)FROM pg_class, moments, emotionsWHERE moments.tableoid = pg_class.oidAND emotions.inserted > 'yesterday'AND moments.inserted BETWEEN 'yesterday' AND 'today'AND emotions.moment_id = moments.moment_idGROUP BY relname, emotionORDER BY relname, emotion;That was a bit faster, but still very slow. Here's the EXPLAIN: http://explain.depesz.com/s/ZdFOn Tue, Jan 31, 2012 at 2:53 PM, Alessandro Gagliardi <alessandro@path.com> wrote:I changed the query a bit so the results would not change over the
course of the day to:WHERE moments.inserted BETWEEN 'yesterday' AND 'today' AND
SELECT relname, emotion, COUNT(feedback_id) FROM pg_class, moments
JOIN emotions USING (moment_id)moments.tableoid = pg_class.oid
GROUP BY relname, emotion ORDER BY relname, emotion;
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