Re: Compression and on-disk sorting

On Fri, May 19, 2006 at 09:03:31AM -0400, Tom Lane wrote:
> Martijn van Oosterhout <kleptog(at)svana(dot)org> writes:
> > I'm seeing 250,000 blocks being cut down to 9,500 blocks. That's almost
> > unbeleiveable. What's in the table?
>
> Yeah, I'd tend to question the test data being used. gzip does not do
> that well on typical text (especially not at the lower settings we'd
> likely want to use).

However, postgres tables are very highly compressable, 10-to-1 is not
that uncommon. pg_proc and pg_index compress by that for example.
Indexes compress even more (a few on my system compress 25-to-1 but
that could just be slack space, the record being 37-to-1
(pg_constraint_conname_nsp_index)).

The only table on my test system over 32KB that doesn't reach 2-to-1
compression with gzip -3 is one of the toast tables.

So getting 25-to-1 is a lot, but possibly not that extreme.
pg_statistic, which is about as close to random data as you're going to
get on a postgres system, compresses 5-to-1.

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.