Re: Compression and on-disk sorting

On Fri, May 19, 2006 at 09:29:44PM +0200, Martijn van Oosterhout wrote:
> On Fri, May 19, 2006 at 10:02:50PM +0300, Hannu Krosing wrote:
> > > > It's just SELECT count(*) FROM (SELECT * FROM accounts ORDER BY bid) a;
> > > > If the tape routines were actually storing visibility information, I'd
> > > > expect that to be pretty compressible in this case since all the tuples
> > > > were presumably created in a single transaction by pgbench.
> >
> > Was he not using pg_bench data ?
>
> Hmm, so there was only 3 integer fields and one varlena structure which
> was always empty. This prepended with a tuple header with mostly blank
> fields or at least repeated, yes, I can see how we might get a 25-to-1
> compression.
>
> Maybe we need to change pgbench so that it puts random text in the
> filler field, that would at least put some strain on the compression
> algorithm...

Wow, I thought there was actually something in there...

True random data wouldn't be such a great test either; what would
probably be best is a set of random words, since in real life you're
unlikely to have truely random data.
--
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