Re: Select Last n Rows Matching an Index Condition (and caches)

You might unknowingly be asking a FAQ. See the end of this section:

http://www.postgresql.org/docs/faqs.FAQ.html#4.6

Depending on the version of postgres you're running and the data types
of the symbol and source columns, you might need to quote (or cast)
your constant data for symbol and source, e.g.:

SELECT *
FROM article
WHERE symbol='12646'
AND source = '19'
ORDER BY time DESC
LIMIT 1000;

-tfo

--
Thomas F. O'Connell
Co-Founder, Information Architect
Sitening, LLC
http://www.sitening.com/
110 30th Avenue North, Suite 6
Nashville, TN 37203-6320
615-260-0005

On Mar 18, 2005, at 3:56 AM, Alex Stapleton wrote:

> Woops sorry we have indexes on (symbol, source, time) and there is no
> date
> column :/
>
> SELECT * FROM article WHERE symbol=12646 AND source = 19 ORDER BY time
> DESC
> LIMIT 1000;
> QUERY PLAN
> -----------------------------------------------------------------------
> -----
> ----------------------------------------
> Limit (cost=321163.29..321165.79 rows=1000 width=466) (actual
> time=33243.924..33246.021 rows=1000 loops=1)
> -> Sort (cost=321163.29..321402.31 rows=95609 width=466) (actual
> time=33243.917..33244.626 rows=1000 loops=1)
> Sort Key: "time"
> -> Seq Scan on article (cost=0.00..301724.00 rows=95609
> width=466) (actual time=0.022..32979.685 rows=42959 loops=1)
> Filter: ((symbol = 12646) AND (source = 19))
> Total runtime: 33258.706 ms
> (6 rows)
>
>
> explain analyze SELECT * FROM article WHERE symbol=12646 AND source =
> 19;
> QUERY PLAN
> -----------------------------------------------------------------------
> -----
> ----------------------------------------
> Seq Scan on article (cost=0.00..301724.00 rows=95609 width=466)
> (actual
> time=0.021..33275.433 rows=42959 loops=1)
> Filter: ((symbol = 12646) AND (source = 19))
> Total runtime: 33320.920 ms
> (3 rows)
>
> We can't use CLUSTER because we need the DB up all the time.
>
> The analyze suggests that it's the I/O taking most of the time to me.
>
>
> -----Original Message-----
> From: Alban Hertroys [mailto:alban(at)magproductions(dot)nl]
> Sent: 18 March 2005 09:48
> To: Alex Stapleton
> Cc: pgsql-general(at)postgresql(dot)org
> Subject: Re: [GENERAL] Select Last n Rows Matching an Index Condition
> (and caches)
>
>
> Alex Stapleton wrote:
>> SELECT * FROM article WHERE symbol=12646 AND source = 19 ORDER BY time
> DESC
>> LIMIT 1000;
>>
>> To get the latest 1000 rows for that symbol and source.
>
>> My (not yet implemented) solution to this problem is to add a
>> SEQUENCE and
>> index it so that by adding a WHERE id > [max_id]-1000 and ordering by
>> time
>> DESC will reduce the I/O quite a lot. Am I right here? It would be
>> nice if
>> there was a way to get PostgreSQL to try and precache the tables
>> pages as
>> well, is there anyway I could achieve something like that? I have
>> toyed
> with
>> creating a ramdisk to store a lookup table of sorts on (we only care
>> about
> a
>> few columns initially) to speed this up a bit but its a right pain in
>> the
>> arse to do by the looks of things.
>
> First question that always gets asked here: What's the output of
> explain
> analyse? Without that, people here can't see where the slowdown is.
>
> I expect though, that the problem is the ordering by time. I imagine
> that you could create an index on time, maybe truncated to months or
> something similar (You can create indices based on functions). That
> index alone should speed up the ordering already.
> It could also be used to cluster the table, which should speed up
> things
> some more, I suppose.
>
> --
> Alban Hertroys
> MAG Productions
>
> T: +31(0)53 4346874
> F: +31(0)53 4346876
> E: alban(at)magproductions(dot)nl
> W: http://www.magproductions.nl
>
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 3: if posting/reading through Usenet, please send an appropriate
> subscribe-nomail command to majordomo(at)postgresql(dot)org so that your
> message can get through to the mailing list cleanly