From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>,
        Andres Freund <andres@anarazel.de>,
        Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>,
        Rajkumar Raghuwanshi <rajkumar.raghuwanshi@enterprisedb.com>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Expression errors with "FOR UPDATE" and postgres_fdw with
 partition wise join enabled.
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Wed, 25 Jul 2018 16:27:41 -0400"
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Date: Wed, 25 Jul 2018 16:42:41 -0400
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Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Jul 24, 2018 at 7:51 AM, Etsuro Fujita
> <fujita.etsuro@lab.ntt.co.jp> wrote:
>> I'm not sure that's a good idea, because I think we have a trade-off
>> relation; the more we make create_plan simple, the more we need to make
>> earlier states of the planner complicated.
>> 
>> And it looks to me like the partitionwise join code is making earlier (and
>> later) stages of the planner too complicated, to make create_plan simple.

> I think that create_plan is *supposed* to be simple.  Its purpose is
> to prune away data that's only needed during planning and add things
> that can be computed at the last minute which are needed at execution
> time.  Making it do anything else is, in my opinion, not good.

I tend to agree with Robert on this.  In particular, if you put anything
into create_plan or later that affects cost estimates, you're really doing
it wrong, because changing those estimates might've changed the plan
entirely.  (As I recall, we have a couple of cheats like that now,
associated with dropping no-op projection and subquery scan nodes.
But let's not add more.)

TBH, I think this entire discussion is proving what sheer folly it was
to allow partitions to have rowtypes not identical to their parent.
But I suppose we're stuck with that now.

			regards, tom lane