From: | "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com> |
---|---|
To: | Zeugswetter Andreas DCP SD <ZeugswetterA(at)spardat(dot)at> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Compression and on-disk sorting |
Date: | 2006-05-16 17:23:33 |
Message-ID: | 20060516172333.GG26212@pervasive.com |
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Lists: | pgsql-hackers |
On Tue, May 16, 2006 at 09:24:38AM +0200, Zeugswetter Andreas DCP SD wrote:
>
> > > Given that any time that happens we end up caring much less about
> CPU
> > > usage and much more about disk IO, for any of these cases that use
> > > non-random access, compressing the data before sending it to disk
> would
> > > potentially be a sizeable win.
> >
> > Note however that what the code thinks is a spill to disk and what
> > actually involves disk I/O are two different things. If you think
> > of it as a spill to kernel disk cache then the attraction is a lot
> > weaker...
>
> Yes, that is very true. However it would also increase the probability
> that spill to disk is not needed, since more data fits in RAM.
That's a pretty thin margin though, depending on how good the
compression is. This also assumes that you have a compression algorithm
that supports random access.
--
Jim C. Nasby, Sr. Engineering Consultant jnasby(at)pervasive(dot)com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
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