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Re: bitmap-index-scan slower than normal index scan

From: Andreas Kretschmer <akretschmer(at)spamfence(dot)net>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: bitmap-index-scan slower than normal index scan
Date: 2007-07-11 20:19:58
Message-ID: 20070711201958.GA18449@KanotixBox (view raw or flat)
Thread:
Lists: pgsql-performance
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> schrieb:


Thanks you and Alex for the response.

> > PostgreSQL 8.1.4 on i386-pc-linux-gnu, compiled by GCC cc (GCC) 3.3.5 (Debian 1:3.3.5-13)
> 
> You need a newer one.

I know ;-)

> 
> This is simply a stupid choice on the part of choose_bitmap_and() ---
> it's adding on a second index to try to filter on maschine when that
> scan will actually just increase the cost.
> 
> I've revisited choose_bitmap_and() a couple times since then; try
> 8.1.9 and see if it gets this right.

Okay, but later.

> 
> Also, part of the problem here looks to be an overestimate of the number
> of rows matching ab = 347735.  It might help to increase the statistics
> target for that column.

I will try this tomorrow and inform you about the result. I've never
done this before, i need to read the docs about this.

Thank you again.


Andreas
-- 
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."    (unknow)
Kaufbach, Saxony, Germany, Europe.              N 51.05082°, E 13.56889°

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