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Re: join removal

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Greg Stark <stark(at)mit(dot)edu>, "<pgsql-hackers(at)postgresql(dot)org>" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: join removal
Date: 2010-03-28 18:04:05
Message-ID: 24817.1269799445@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Sun, Mar 28, 2010 at 12:19 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> * I left join_is_removable where it was, mainly so that it was easy to
>> compare how much it changed for this usage (not a lot). I'm not sure
>> that joinpath.c is an appropriate place for it anymore, though I can't
>> see any obviously better place either. Any thoughts on that?

> I dislike the idea of leaving it in joinpath.c.  I don't even think it
> properly belongs in the path subdirectory since it no longer has
> anything to do with paths.  Also worth thinking about where we would
> put the logic I pontificated about here:
> http://archives.postgresql.org/pgsql-hackers/2009-10/msg01012.php

The only argument I can see for leaving it where it is is that it
depends on clause_sides_match_join, which we'd have to either duplicate
or global-ize in order to continue sharing that code.  However, since
join_is_removable now needs a slightly different API for that anyway
(cf changes in draft patch), it's probably better to not try to share it.
So let's put the join removal code somewhere else.  The reasonable
alternatives seem to be:

* in a new file in prep/.  Although this clearly has the flavor of
preprocessing, all the other work in prep/ is done before we get into
query_planner().  So this choice seems a bit dubious.

* directly in plan/planmain.c.  Has the advantage of being where the
caller is, so no globally visible function declaration needed.  No other
redeeming social value though.

* in plan/initsplan.c.  Somewhat reasonable, although that file is
rather large already.

* in a new file in plan/.  Not sure if it's worth this, though your
thought that we might add more logic later makes it more defensible.

Comments?

			regards, tom lane

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