Re: Estimating space required for indexes

On Mon, 28 Apr 2003, Tom Lane wrote:

> Oleg Bartunov <oleg@sai.msu.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@sai.msu.su, http://www.sai.msu.su/~megera/
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