Re: Compression and on-disk sorting

On Wed, May 17, 2006 at 11:38:05AM -0400, Tom Lane wrote:
> "Jim C. Nasby" <jnasby@pervasive.com> writes:
> > What *might* make sense would be to provide two locations for pgsql_tmp,
> > because a lot of operations in there involve reading and writing at the
> > same time:
> 
> > Read from heap while writing tapes to pgsql_tmp
> > read from tapes while writing final version to pgsql_tmp
> 
> Note that a large part of the reason for the current logtape.c design
> is to avoid requiring 2X or more disk space to sort X amount of data.
> AFAICS, any design that does the above will put us right back in the 2X
> regime.  That's a direct, measurable penalty; it'll take more than
> handwaving arguments to convince me we should change it in pursuit of
> unquantified speed benefits.

I certainly agree that there's no reason to make this change without
testing, but if you've got enough spindles laying around to actually
make use of this I suspect that requiring 2X the disk space to sort X
won't bother you.

Actually, I suspect in most cases it won't matter; I don't think people
make a habit of trying to sort their entire database. :) But we'd want
to protect for the oddball cases... yech.
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461