From: Tom Lane <tgl@sss.pgh.pa.us>
To: Julien Rouhaud <rjuju123@gmail.com>
cc: thibaut.madelaine@dalibo.com, Robert Haas <robertmhaas@gmail.com>,
        PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Subject: Re: BUG #15669: Error with unnest in PG 11 (ERROR: 0A000)
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Comments: In-reply-to Julien Rouhaud <rjuju123@gmail.com>
	message dated "Tue, 05 Mar 2019 17:34:21 +0100"
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Date: Tue, 05 Mar 2019 12:28:57 -0500
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Julien Rouhaud <rjuju123@gmail.com> writes:
> On Tue, Mar 5, 2019 at 4:39 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Hmm, that's definitely a bug.  It looks like we're forgetting to make
>> a ProjectSet plan node for the unnest() if we realize that the query
>> is a no-op; but I'm not sure why 10.x doesn't have the same issue.
>> Digging ...

> It seems to be due to 11cf92f6e2e which bypass adjust_paths_for_srfs()
> in case of dummy rel.  I'm not familiar with that this code, but
> attached patch seems to fix the issue without breaking regression
> tests.

Hm, yeah, that function is definitely a few bricks shy of a load.
The amount of code it's bypassing for the dummy-rel case is pretty
scary.  While it doesn't look like anything except the SRF case is
actively broken, there's a lot of room there for somebody to insert
new code and not notice that they need to fix the dummy-rel path
as well.  Having to duplicate the adjust_paths_for_srfs call doesn't
bode well for the future.

I'm inclined to fix it by not having the early-return path, but rather

	if (is_dummy_rel)
	{
		// minimum possible amount of code here
	}
	else
	{
		// minimum possible amount of code here, too
	}

	// maximum possible amount of code here

			regards, tom lane