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Re: WIP: cross column correlation ...

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, PostgreSQL - Hans-Jürgen Schönig <postgres(at)cybertec(dot)at>, pgsql-hackers Hackers <pgsql-hackers(at)postgresql(dot)org>, Boszormenyi Zoltan <zb(at)cybertec(dot)at>
Subject: Re: WIP: cross column correlation ...
Date: 2011-02-24 03:30:08
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Robert Haas wrote:
> If you want to take the above as in any way an exhaustive survey of
> the landscape (which it isn't), C seems like a standout, maybe
> augmented by the making the planner able to notice that A1 = x1 AND A2
> = x2 is equivalent to (A1,A2) = (x1, x2) so you don't have to rewrite
> queries as much.
> I don't really know how to handle the join selectivity problem.  I am
> not convinced that there is a better solution to that than decorating
> the query.  After all the join selectivity depends not only on the
> join clause itself, but also on what you've filtered out of each table
> in the meantime.

Thinking some more, I think another downside to the "decorate the query"
idea is that many queries use constants that are supplied only at
runtime, so there would be no way to hard-code a selectivity value into
a query when you don't know the value.  Could a selectivity function
handle that?

  Bruce Momjian  <bruce(at)momjian(dot)us>

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

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