Fix plpgsql's handling of simple expressions in scrollable cursors.

exec_save_simple_expr did not account for the possibility that
standard_planner would stick a Materialize node atop the plan
of even a simple Result, if CURSOR_OPT_SCROLL is set.  This led
to an "unexpected plan node type" error.

This is a very old bug, but it'd only be reached by declaring a
cursor for a "SELECT simple-expression" query and explicitly
marking it scrollable, which is an odd thing to do.  So the lack
of prior reports isn't too surprising.

Bug: #18859
Reported-by: Olleg Samoylov <splarv@ya.ru>
Author: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/18859-0d5f28ac99a37059@postgresql.org
Backpatch-through: 13

src/pl/plpgsql/src/expected/plpgsql_simple.out

@@ -118,3 +118,14 @@ select simplecaller();
            44
 (1 row)
 
+-- Check handling of simple expression in a scrollable cursor (bug #18859)
+do $$
+declare
+ p_CurData refcursor;
+ val int;
+begin
+ open p_CurData scroll for select 42;
+ fetch p_CurData into val;
+ raise notice 'val = %', val;
+end; $$;
+NOTICE:  val = 42

src/pl/plpgsql/src/pl_exec.c

@@ -8302,10 +8302,12 @@ exec_save_simple_expr(PLpgSQL_expr *expr, CachedPlan *cplan)
 	/*
 	 * Ordinarily, the plan node should be a simple Result.  However, if
 	 * debug_parallel_query is on, the planner might've stuck a Gather node
-	 * atop that.  The simplest way to deal with this is to look through the
-	 * Gather node.  The Gather node's tlist would normally contain a Var
-	 * referencing the child node's output, but it could also be a Param, or
-	 * it could be a Const that setrefs.c copied as-is.
+	 * atop that; and/or if this plan is for a scrollable cursor, the planner
+	 * might've stuck a Material node atop it.  The simplest way to deal with
+	 * this is to look through the Gather and/or Material nodes.  The upper
+	 * node's tlist would normally contain a Var referencing the child node's
+	 * output, but it could also be a Param, or it could be a Const that
+	 * setrefs.c copied as-is.
 	 */
 	plan = stmt->planTree;
 	for (;;)
@@ -8323,7 +8325,7 @@ exec_save_simple_expr(PLpgSQL_expr *expr, CachedPlan *cplan)
 				   ((Result *) plan)->resconstantqual == NULL);
 			break;
 		}
-		else if (IsA(plan, Gather))
+		else if (IsA(plan, Gather) || IsA(plan, Material))
 		{
 			Assert(plan->lefttree != NULL &&
 				   plan->righttree == NULL &&

src/pl/plpgsql/src/sql/plpgsql_simple.sql

@@ -102,3 +102,15 @@ as $$select 22 + 22$$;
 select simplecaller();
 
 select simplecaller();
+
+-- Check handling of simple expression in a scrollable cursor (bug #18859)
+
+do $$
+declare
+ p_CurData refcursor;
+ val int;
+begin
+ open p_CurData scroll for select 42;
+ fetch p_CurData into val;
+ raise notice 'val = %', val;
+end; $$;