Re: alternative compression algorithms?

Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Apr 20, 2015 at 9:03 AM, Tomas Vondra
> <tomas.vondra@2ndquadrant.com> wrote:
>> Sure, it's not an ultimate solution, but it might help a bit. I do have
>> other ideas how to optimize this, but in the planner every milisecond
>> counts. Looking at 'perf top' and seeing pglz_decompress() in top 3.

> I suggested years ago that we should not compress data in
> pg_statistic.  Tom shot that down, but I don't understand why.  It
> seems to me that when we know data is extremely frequently accessed,
> storing it uncompressed makes sense.

I've not been following this thread, but I do not think your argument here
holds any water.  pg_statistic entries are generally fetched via the
syscaches, and we fixed things years ago so that toasted tuple entries
are detoasted before insertion in syscache.  So I don't believe that
preventing on-disk compression would make for any significant improvement,
at least not after the first reference within a session.

Also, it's a very long way from "some pg_statistic entries are frequently
accessed" to "all pg_statistic entries are frequently accessed".
        regards, tom lane