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Re: Path question

From: Greg Stark <gsstark(at)mit(dot)edu>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Hans-Jürgen Schönig <hs(at)cybertec(dot)at>, Boszormenyi Zoltan <zb(at)cybertec(dot)at>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Path question
Date: 2010-09-30 00:46:51
Message-ID: AANLkTinhiRcAmVL+rZQJb-JCOup8Sk2=eSkvs2M8ZP96@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
2010/9/23 Robert Haas <robertmhaas(at)gmail(dot)com>:

> All of this leaves me wondering why Greg ended up ifdefing out this
> code in the first place.  There's probably something I'm missing
> here...  but for now I can't think of a better idea than just removing
> the #ifdefs and hoping that whatever problem they were causing was
> limited to an earlier version of the code that no longer exists.
>

Sorry, I haven't gone completely AWOL, I just hadn't noticed this
thread. pgsql-hackers turns out to be kind of a lot of traffic if
you're not reading it continuously.

As you subsequently discovered I added these FIXMEs because without
them the paths just didn't compare equal when it was comparing the
paths of the children with the paths of the parent.

I think the reason you had difficulty demonstrating problems with at
least some of the FIXMEs was that they really aren't functionally
necessary. They're there because when Tom implemented the equivalence
classes he had a clear idea of what they were supposed to represent
and what variables they needed to include to represent that. And
including variables of child relations or subqueries of a UNION in an
equivalence class representing the parent relation just didn't fit
within that. It doesn't necessarily cause problems but it changes the
data structure representation invariant from what he had in mind.

I don't have a good grasp of exactly what the implications of changing
this data structure rep invariant are though. Is having hundreds or
thousands of variables in a single equivalence class (actually for a
whole bunch if the pathkey list is long) going to cause performance
problems? Is including variables that are only present for one child
of the relation going to limit the usefulness of the equivalence class
data structure for solving other problems where those columns really
aren't equivalent? Also, since I don't really understand what's going
on I wondered if there wasn't an obvious way to accomplish the same
thing perhaps more efficiently.



-- 
greg

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