From: Tom Lane <tgl@sss.pgh.pa.us>
To: Dmitry Dolgov <9erthalion6@gmail.com>
cc: "Poot, Bas (B.J.)" <bas.poot@politie.nl>,
        "pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
Subject: Re: XX000: unknown type of jsonb container.
In-reply-to: <2433209.1622416353@sss.pgh.pa.us>
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Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Sun, 30 May 2021 19:12:33 -0400"
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Date: Sun, 30 May 2021 20:59:28 -0400
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I wrote:
> I'm not sure about creating a test case.  The case reported here is
> far too expensive to use in the regression tests.

Ah, we can do it in select_parallel, which reduces all the parallel
costs to zero to favor these sorts of plans.

This test case only fails back to v13, as does the original example.
I suspect we should back-patch the code change further though,
since create_projection_plan will be just as confused by such cases
in earlier branches.

			regards, tom lane


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diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer=
/util/pathnode.c
index b248b038e0..9ce5f95e3b 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -2632,7 +2632,23 @@ create_projection_path(PlannerInfo *root,
 					   PathTarget *target)
 {
 	ProjectionPath *pathnode =3D makeNode(ProjectionPath);
-	PathTarget *oldtarget =3D subpath->pathtarget;
+	PathTarget *oldtarget;
+
+	/*
+	 * We mustn't put a ProjectionPath directly above another; it's useless
+	 * and will confuse create_projection_plan.  Rather than making sure all
+	 * callers handle that, let's implement it here, by stripping off any
+	 * ProjectionPath in what we're given.  Given this rule, there won't be
+	 * more than one.
+	 */
+	if (IsA(subpath, ProjectionPath))
+	{
+		ProjectionPath *subpp =3D (ProjectionPath *) subpath;
+
+		Assert(subpp->path.parent =3D=3D rel);
+		subpath =3D subpp->subpath;
+		Assert(!IsA(subpath, ProjectionPath));
+	}
 =

 	pathnode->path.pathtype =3D T_Result;
 	pathnode->path.parent =3D rel;
@@ -2658,6 +2674,7 @@ create_projection_path(PlannerInfo *root,
 	 * Note: in the latter case, create_projection_plan has to recheck our
 	 * conclusion; see comments therein.
 	 */
+	oldtarget =3D subpath->pathtarget;
 	if (is_projection_capable_path(subpath) ||
 		equal(oldtarget->exprs, target->exprs))
 	{
diff --git a/src/test/regress/expected/select_parallel.out b/src/test/regr=
ess/expected/select_parallel.out
index 05ebcb284a..4ea1aa7dfd 100644
--- a/src/test/regress/expected/select_parallel.out
+++ b/src/test/regress/expected/select_parallel.out
@@ -1126,6 +1126,29 @@ ORDER BY 1, 2, 3;
 ------------------------------+---------------------------+-------------+=
--------------
 (0 rows)
 =

+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT generate_series(1, two), array(select generate_series(1, two))
+  FROM tenk1 ORDER BY tenthous;
+                              QUERY PLAN                              =

+----------------------------------------------------------------------
+ ProjectSet
+   Output: generate_series(1, tenk1.two), (SubPlan 1), tenk1.tenthous
+   ->  Gather Merge
+         Output: tenk1.two, tenk1.tenthous
+         Workers Planned: 4
+         ->  Result
+               Output: tenk1.two, tenk1.tenthous
+               ->  Sort
+                     Output: tenk1.tenthous, tenk1.two
+                     Sort Key: tenk1.tenthous
+                     ->  Parallel Seq Scan on public.tenk1
+                           Output: tenk1.tenthous, tenk1.two
+   SubPlan 1
+     ->  ProjectSet
+           Output: generate_series(1, tenk1.two)
+           ->  Result
+(16 rows)
+
 -- test passing expanded-value representations to workers
 CREATE FUNCTION make_some_array(int,int) returns int[] as
 $$declare x int[];
diff --git a/src/test/regress/sql/select_parallel.sql b/src/test/regress/s=
ql/select_parallel.sql
index d31e290ec2..f924731248 100644
--- a/src/test/regress/sql/select_parallel.sql
+++ b/src/test/regress/sql/select_parallel.sql
@@ -429,6 +429,10 @@ ORDER BY 1;
 SELECT * FROM information_schema.foreign_data_wrapper_options
 ORDER BY 1, 2, 3;
 =

+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT generate_series(1, two), array(select generate_series(1, two))
+  FROM tenk1 ORDER BY tenthous;
+
 -- test passing expanded-value representations to workers
 CREATE FUNCTION make_some_array(int,int) returns int[] as
 $$declare x int[];

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