Thank you for a good workaround.
Even BETTER would be to fix the aggregates so workarounds wouldn't have to
be found.
Thanks again,
L.
On Fri, 13 Dec 2002, Josh Berkus wrote:
>
>
> ---------- Forwarded Message ----------
>
> Subject: Re: [PERFORM] Odd Sort/Limit/Max Problem
> Date: Fri, 13 Dec 2002 12:10:20 -0800 (PST)
> From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
> To: Josh Berkus <josh@agliodbs.com>
> Cc: <pgsql-performance@postgresql.org>
>
> On Fri, 13 Dec 2002, Josh Berkus wrote:
>
> > First, as expected, a regular aggregate is slow:
>
> > So we use the workaround standard for PostgreSQL:
> >
> > ... which is fast, but returns NULL, since nulls sort to the bottom! So we
> > add IS NOT NULL:
> >
> > jwnet=> explain analyze select date_resolved from case_clients where
> > date_resolved is not null order by date_resolved desc limit 1;
> > NOTICE: QUERY PLAN:
> >
> > Limit (cost=0.00..4.06 rows=1 width=4) (actual time=219.63..219.64 rows=1
> > loops=1)
> > -> Index Scan Backward using idx_caseclients_resolved on case_clients
> > (cost=0.00..163420.59 rows=40272 width=4) (actual time=219.62..219.62 rows=2
> > loops=1)
> > Total runtime: 219.76 msec
> >
> > Aieee! Almost as slow as the aggregate!
>
> I'd suggest trying a partial index on date_resolved where date_resolve is
> not null. In my simple tests on about 200,000 rows of ints where 50% are
> null that sort of index cut the runtime on my machine from 407.66 msec to
> 0.15 msec.
>
>
>
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--
Laurette Cisneros
The Database Group
(510) 420-3137
NextBus Information Systems, Inc.
www.nextbus.com
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