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Re: Estimating space required for indexes

From: Oleg Bartunov <oleg(at)sai(dot)msu(dot)su>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Shridhar Daithankar <shridhar_daithankar(at)persistent(dot)co(dot)in>,pgsql-general(at)postgresql(dot)org
Subject: Re: Estimating space required for indexes
Date: 2003-04-28 15:16:45
Message-ID: Pine.GSO.4.55.0304281911450.13875@ra.sai.msu.su (view raw or flat)
Thread:
Lists: pgsql-general
On Mon, 28 Apr 2003, Tom Lane wrote:

> Oleg Bartunov <oleg(at)sai(dot)msu(dot)su> writes:
> >> But this is not the whole story because heap pages are normally crammed
> >> full while btree index pages are normally only filled 2/3rds full during
> >> initial creation.  (Plus you have to allow for upper b-tree levels, but
>
> > Are there any benefits from getting btree index pages to be more effective
> > in space usage ? I've read some paper about 98% space usage for Btree.
>
> Standard theory says that optimal load for a b-tree is 65-70%.  We used
> to make CREATE INDEX cram the leaf pages full, but that just resulted in
> a lot of page splits as soon as you did any inserts or updates.  And the
> page splits destroy the physical ordering of the index, which negates
> any I/O savings you might have had from fewer pages.

Thanks for comment, I didn't think about that.

>
> I suppose if you know that the table will be static there might be some
> value in telling CREATE INDEX to pack the index pages full, but I'm not
> sure it's worth the trouble ...
>

Sure.

> 			regards, tom lane
>

	Regards,
		Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg(at)sai(dot)msu(dot)su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83


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