From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Proposed refactoring of planner header files
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Mon, 28 Jan 2019 15:33:10 -0500"
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Date: Mon, 28 Jan 2019 16:51:11 -0500
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Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Jan 28, 2019 at 3:17 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I'm really unhappy that force_parallel_mode and
>> parallel_leader_participation are being treated as planner GUCs.

> The only use of parallel_leader_participation at plan time seems to be
> to twiddle the costing, and the use of it in the executor is to decide
> whether to have the leader participate.  So if the values differ,
> you'll get a plan running a behavior for which plan selection was not
> optimized.  I don't know whether it's useful to intentionally allow
> this so that you can see how the same plan behaves under the other
> setting, or whether it's just a wart we'd be better off without.  It
> might be confusing, though, if you change the setting and it doesn't
> force a replan.

Well, that puts it at the ill-considered end of the spectrum instead
of the outright-broken end, but I still say it's a bad idea.  Planner
GUCs ought to control the produced plan, not other behaviors.

			regards, tom lane