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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Juan José Santamaría Flecha <juanjo.santamaria@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: GEQO and join_collapse_limit correlation
Date: Fri, 05 Jan 2018 15:29:16 -0500
Message-ID: <12214.1515184156@sss.pgh.pa.us> (raw)
In-Reply-To: <CAC+AXB3ZaGLc=1qJgbWHGtguw4Nd9XgGeqEeP5-K0OSbeCNgGA@mail.gmail.com>
References: <CAC+AXB3Ug25QUT+hnoZ2bk6w5oFfBbk1qaHu63G1qKm2NO=z1A@mail.gmail.com>
	<29566.1515168977@sss.pgh.pa.us>
	<CAC+AXB3ZaGLc=1qJgbWHGtguw4Nd9XgGeqEeP5-K0OSbeCNgGA@mail.gmail.com>

=?UTF-8?Q?Juan_Jos=C3=A9_Santamar=C3=ADa_Flecha?= <juanjo.santamaria@gmail.com> writes:
> In an environment with geqo_threshold=1 and join_collapse_limit=1, would
> the planner be GEQO exclusive (and syntactic)?

GEQO's only function, basically, is to search for the join order to use.
If you're constraining the join order completely with
join_collapse_limit=1 then forcing the GEQO path to be taken would just
add pointless overhead.  (If it does anything at all ... I don't remember
the logic exactly but we might be bright enough not to bother with GEQO in
such a situation, regardless of geqo_threshold.)

			regards, tom lane

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