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Re: [HACKERS] choose_bitmap_and again (was Re: [PERFORM] Strangely Variable Query Performance)

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Steve <cheetah(at)tanabi(dot)org>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-patches(at)postgreSQL(dot)org
Subject: Re: [HACKERS] choose_bitmap_and again (was Re: [PERFORM] Strangely Variable Query Performance)
Date: 2007-04-16 23:01:04
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Lists: pgsql-hackerspgsql-patchespgsql-performance
Steve wrote:

> >Can you find any cases where it makes a worse choice than before?
> >Another thing to pay attention to is whether the planning time gets
> >noticeably worse.  If we can't find any cases where it loses badly
> >on those measures, I'll feel comfortable in applying it...
> 	Okay, here's the vedict; all the "extremely slow" queries (i.e. 
> queries that took more than 30 seconds and upwards of several minutes to 
> complete) are now running in the realm of reason.  In fact, most queries 
> that took between 1 and 4 minutes are now down to taking about 9 seconds 
> which is obviously a tremendous improvement.
> 	A few of the queries that were taking 9 seconds or less are 
> "slightly slower" -- meaning a second or two slower.  However most of them 
> are running at the same speed they were before, or better.
> 	So I'd say as far as I can tell with my application and my 
> dataset, this change is solid and an obvious improvement.

Maybe it would be interesting to see in detail those cases that got a
bit slower, to further tweak the heuristic if necessary.  Is the extra
time, time spent in planning or in execution?

Alvaro Herrera                      
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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