From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
cc: feasiblechart@gmail.com, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #19742: `INTERSECT` under a `UNION ALL` with an empty arm
 fails with "could not find pathkey item t"
In-reply-to: <290582.1791092578@sss.pgh.pa.us>
References: <19742-dc403ca277cad1d3@postgresql.org>
 <261146.1791064428@sss.pgh.pa.us> <273647.1791075609@sss.pgh.pa.us>
 <CAApHDvreoQsZs=CGNz2GjAsHV=j-tb_ycOHW3EFsBp1TO8BxcA@mail.gmail.com>
 <290582.1791092578@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Sun, 04 Oct 2026 01:42:58 -0400"
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Date: Sun, 04 Oct 2026 13:01:39 -0400
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I wrote:
> My own thoughts were along the lines of "don't ever assign pathkeys to
> an AppendPath"; not sure if that's equivalent to your first idea.

Concretely, the attached fixes the given test case.  There are other
calls to create_append_path in prepunion.c, and I think they may
all need to do likewise, but I didn't analyze them.

It'd be nominally cleaner to add a flag to create_append_path telling
it whether it's allowed to override the given pathkeys.  I didn't do
that here because it seems like this is a localized problem that
should eventually be fixed inside prepunion.c, but there's room to
argue differently.

			regards, tom lane


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diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimize=
r/prep/prepunion.c
index b136f12ff3b..c4e95f14dc5 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -862,6 +862,17 @@ generate_union_paths(SetOperationStmt *op, PlannerInf=
o *root,
 	apath =3D (Path *) create_append_path(root, result_rel, cheapest,
 										NIL, NULL, 0, false, -1);
 =

+	/*
+	 * Although we told create_append_path to assign NIL pathkeys to the
+	 * AppendPath, it may have overridden that (if there's just one survivin=
g
+	 * child path, it will use that path's pathkeys).  However, createplan.c
+	 * will fail because the append relation's tlist contains varno-0 Vars
+	 * (cf. generate_append_tlist), which won't match what is in the pathkey=
s.
+	 * We need to fix that someday, but for now, just force the AppendPath's
+	 * pathkeys back to NIL.
+	 */
+	apath->pathkeys =3D NIL;
+
 	/*
 	 * Initialize the result row estimate to the total input size.  This is
 	 * correct for UNION ALL; for the UNION case it is overwritten below wit=
h

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