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Re: WIP: cross column correlation ...

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: PostgreSQL - Hans-Jürgen Schönig <postgres(at)cybertec(dot)at>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers Hackers <pgsql-hackers(at)postgresql(dot)org>, Boszormenyi Zoltan <zb(at)cybertec(dot)at>
Subject: Re: WIP: cross column correlation ...
Date: 2011-02-23 22:37:39
Message-ID: 201102232237.p1NMbdb19397@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
PostgreSQL - Hans-Jrgen Schnig wrote:
> > Those are real problems, but I still want it.  The last time I hit
> > this problem I spent two days redesigning my schema and adding
> > triggers all over the place to make things work.  If I had been
> > dealing with a 30TB database instead of a 300MB database I would have
> > been royally up a creek.
> >
> > To put that another way, it's true that some people can't adjust their
> > queries, but also some people can.  It's true that nonstandard stuff
> > sucks, but queries that don't work suck, too.  And as for better
> > solutions, how many major release cycles do we expect people to wait
> > for them?  Even one major release cycle is an eternity when you're
> > trying to get the application working before your company runs out of
> > money, and this particular problem has had a lot of cycles expended on
> > it without producing anything very tangible (proposed patch, which
> > like you I can't spare a lot of cycles to look at just now, possibly
> > excepted).
> 
> 
> 
> cheapest and easiest solution if you run into this: add "fake" functions
> which the planner cannot estimate properly.  use OR to artificially
> prop up estimates or use AND to artificially lower them. there is
> actually no need to redesign the schema to get around it but it is such
> an ugly solution that it does not even deserve to be called "ugly" ...
> however, fast and reliable way to get around it.

I agree that is super-ugly and we do need to address the cross-column
statistics better.  I personally like the 2-D histogram idea:

	http://archives.postgresql.org/pgsql-hackers/2010-12/msg00913.php

--
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +

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