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Re: Improving our clauseless-join heuristics

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Amit Kapila <amit(dot)kapila(at)huawei(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Improving our clauseless-join heuristics
Date: 2012-04-16 03:39:54
Message-ID: 29472.1334547594@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Amit Kapila <amit(dot)kapila(at)huawei(dot)com> writes:
> I want to clarify small doubt in this regard. 
> In function make_rels_by_clause_joins(..), it tries to join the given
> relation old_rel with other relations if there exist a join between them. 
> What I can understand is, it is because if there exists a join condition its
> better to join with that relation.
> However if the given relation old_rel is not able to join any relation, then
> why can't it try to make cross-join with other relations there itself.

Hm?  That case is handled by make_rels_by_clauseless_joins.  I suppose
we could refactor to combine those two functions, but what's the point?
If that's not what your question is about, then I don't understand.

			regards, tom lane

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