Re: Compressing table images

On Thu, May 11, 2006 at 05:05:26PM -0400, Alvaro Herrera wrote:
> Joshua D. Drake wrote:
> > Brian Hurt wrote:
> > >My apologies if this subject has already been hashed to death, or if 
> > >this is the wrong list, but I was wondering if people had seen this paper:
> > >http://www.cwi.nl/htbin/ins1/publications?request=intabstract&key=ZuHeNeBo:ICDE:06 
> > >
> > >
> > >Basically it describes a compression algorithm for tables of a 
> > >database.  The huge advantage of  doing this is that it reduced the disk 
> > >traffic by (approximately) a factor of four- at the cost of more CPU 
> > >utilization.
> > >Any thoughts or comments?
> > 
> > I don't know if that is the algorithm we use but PostgreSQL will 
> > compress its data within the table.
> 
> But only in certain very specific cases.  And we compress on a
> per-attribute basis.  Compressing at the page level is pretty much out
> of the question; but compressing at the tuple level I think is doable.
> How much benefit that brings is another matter.  I think we still have
> more use for our limited manpower elsewhere.

Except that I think it would be highly useful to allow users to change
the limits used for both toasting and compressing on a per-table and/or
per-field basis. For example, if you have a varchar(1500) in a table
it's unlikely to ever be large enough to trigger toasting, but if that
field is rarely updated it could be a big win to store it toasted. Of
course you can always create a 'side table' (vertical partitioning), but
all of that framework already exists in the database; we just don't
provide the required knobs. I suspect it wouldn't be that hard to expose
those knobs. In fact, if we could agree on syntax, this is probably a
beginner TODO.

ISTR having this discussion on one of the lists recently, but I can't
find it, and don't see anything in the TODO. Basically, I think we'd
want knobs that say: if this field is over X size, compress it. If it's
over Y size, store it externally. Per-table and per-cluster (ie: GUC)
knobs for that would be damn handy as well.
-- 
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