Re: XX000: iso-8859-1 type of jsonb container.

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

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 = makeNode(ProjectionPath);
-    PathTarget *oldtarget = 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 = (ProjectionPath *) subpath;
+
+        Assert(subpp->path.parent == rel);
+        subpath = subpp->subpath;
+        Assert(!IsA(subpath, ProjectionPath));
+    }

     pathnode->path.pathtype = T_Result;
     pathnode->path.parent = 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 = 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/regress/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/sql/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[];