do_some_plan_validation_during_create_plan.patch

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index cd6d72c763..56e28ba739 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -313,6 +313,21 @@ static ModifyTable *make_modifytable(PlannerInfo *root, Plan *subplan,
 									 List *rowMarks, OnConflictExpr *onconflict, int epqParam);
 static GatherMerge *create_gather_merge_plan(PlannerInfo *root,
 											 GatherMergePath *best_path);
+static bool contains_a_parallel_path(Path *path);
+static bool contains_only_parallel_aware_and_safe_paths(Path *path);
+
+/*
+ * PathTypeCount
+ *		Used for various checks to assert plans are sane in assert enabled
+ *		builds.
+ */
+typedef struct PathTypeCount
+{
+	uint64 count;
+	uint64 parallel_safe_count;
+	uint64 parallel_aware_count;
+
+} PathTypeCount;
 
 
 /*
@@ -389,6 +404,10 @@ create_plan_recurse(PlannerInfo *root, Path *best_path, int flags)
 	/* Guard against stack overflow due to overly complex plans */
 	check_stack_depth();
 
+	/* Parallel aware paths should contain only parallel aware subpaths. */
+	Assert(!best_path->parallel_aware ||
+		   contains_only_parallel_aware_and_safe_paths(best_path));
+
 	switch (best_path->pathtype)
 	{
 		case T_SeqScan:
@@ -481,6 +500,13 @@ create_plan_recurse(PlannerInfo *root, Path *best_path, int flags)
 		case T_Gather:
 			plan = (Plan *) create_gather_plan(root,
 											   (GatherPath *) best_path);
+
+			/*
+			 * We expect a Gather to contain at least one parallel path unless
+			 * running in single_copy mode.
+			 */
+			Assert(((GatherPath *) best_path)->single_copy ||
+				   contains_a_parallel_path(((GatherPath *) best_path)->subpath));
 			break;
 		case T_Sort:
 			plan = (Plan *) create_sort_plan(root,
@@ -537,6 +563,8 @@ create_plan_recurse(PlannerInfo *root, Path *best_path, int flags)
 		case T_GatherMerge:
 			plan = (Plan *) create_gather_merge_plan(root,
 													 (GatherMergePath *) best_path);
+			/* We expect a GatherMerge to contain at least one parallel path */
+			Assert(contains_a_parallel_path(((GatherMergePath *) best_path)->subpath));
 			break;
 		default:
 			elog(ERROR, "unrecognized node type: %d",
@@ -7052,6 +7080,210 @@ make_modifytable(PlannerInfo *root, Plan *subplan,
 	return node;
 }
 
+/*
+ * path_tree_walker
+ *		Walk a path tree beginning with 'path' and call the 'walker' function
+ *		for that path and each of its subpaths recursively.
+ */
+static void
+path_tree_walker(Path *path, void (*walker) (), void *context)
+
+{
+	if (path == NULL)
+		return;
+
+	/* Guard against stack overflow due to overly complex path trees */
+	check_stack_depth();
+
+	walker(path, context);
+
+	switch (path->pathtype)
+	{
+		case T_SeqScan:
+		case T_SampleScan:
+		case T_IndexScan:
+		case T_IndexOnlyScan:
+		case T_BitmapHeapScan:
+		case T_TidScan:
+		case T_TidRangeScan:
+		case T_SubqueryScan:
+		case T_FunctionScan:
+		case T_TableFuncScan:
+		case T_ValuesScan:
+		case T_CteScan:
+		case T_WorkTableScan:
+		case T_NamedTuplestoreScan:
+		case T_ForeignScan:
+		case T_CustomScan:
+			/* Scan paths have no subpaths */
+			break;
+		case T_HashJoin:
+		case T_MergeJoin:
+		case T_NestLoop:
+			path_tree_walker(((JoinPath *) path)->outerjoinpath, walker, context);
+			path_tree_walker(((JoinPath *) path)->innerjoinpath, walker, context);
+			break;
+		case T_Append:
+			{
+				AppendPath *apath = (AppendPath *) path;
+				ListCell	   *lc;
+
+				foreach(lc, apath->subpaths)
+				{
+					Path *subpath = lfirst(lc);
+
+					path_tree_walker(subpath, walker, context);
+				}
+			}
+			break;
+			case T_MergeAppend:
+			{
+				MergeAppendPath *mpath = (MergeAppendPath *) path;
+				ListCell	   *lc;
+
+				foreach(lc, mpath->subpaths)
+				{
+					Path *subpath = lfirst(lc);
+
+					path_tree_walker(subpath, walker, context);
+				}
+			}
+			break;
+		case T_Result:
+			if (IsA(path, ProjectionPath))
+			{
+				path_tree_walker(((ProjectionPath *) path)->subpath, walker, context);
+			}
+			else if (IsA(path, MinMaxAggPath))
+			{
+				/* MinMaxAggPath has no subpaths */
+			}
+			else if (IsA(path, GroupResultPath))
+			{
+				/* GroupResultPath has no subpaths */
+			}
+			else
+			{
+				/* No subpaths for any other Result type path */
+			}
+			break;
+		case T_ProjectSet:
+			path_tree_walker(((ProjectSetPath *) path)->subpath, walker, context);
+			break;
+		case T_Material:
+			path_tree_walker(((MaterialPath *) path)->subpath, walker, context);
+			break;
+		case T_Memoize:
+			path_tree_walker(((MemoizePath *) path)->subpath, walker, context);
+			break;
+		case T_Unique:
+			if (IsA(path, UpperUniquePath))
+			{
+				path_tree_walker(((UpperUniquePath *) path)->subpath, walker, context);
+			}
+			else
+			{
+				Assert(IsA(path, UniquePath));
+				path_tree_walker(((UniquePath *) path)->subpath, walker, context);
+			}
+			break;
+		case T_Gather:
+			path_tree_walker(((GatherPath *) path)->subpath, walker, context);
+			break;
+		case T_Sort:
+			path_tree_walker(((SortPath *) path)->subpath, walker, context);
+			break;
+		case T_IncrementalSort:
+			path_tree_walker(((IncrementalSortPath *) path)->spath.subpath, walker, context);
+			break;
+		case T_Group:
+			path_tree_walker(((GroupPath *) path)->subpath, walker, context);
+			break;
+		case T_Agg:
+			if (IsA(path, GroupingSetsPath))
+				path_tree_walker(((GroupingSetsPath *) path)->subpath, walker, context);
+			else
+			{
+				Assert(IsA(path, AggPath));
+				path_tree_walker(((AggPath *) path)->subpath, walker, context);
+			}
+			break;
+		case T_WindowAgg:
+			path_tree_walker(((WindowAggPath *) path)->subpath, walker, context);
+			break;
+		case T_SetOp:
+			path_tree_walker(((SetOpPath *) path)->subpath, walker, context);
+			break;
+		case T_RecursiveUnion:
+			path_tree_walker(((RecursiveUnionPath *) path)->leftpath, walker, context);
+			path_tree_walker(((RecursiveUnionPath *) path)->rightpath, walker, context);
+			break;
+		case T_LockRows:
+			path_tree_walker(((LockRowsPath *) path)->subpath, walker, context);
+			break;
+		case T_ModifyTable:
+			path_tree_walker(((ModifyTablePath *) path)->subpath, walker, context);
+			break;
+		case T_Limit:
+			path_tree_walker(((LimitPath *) path)->subpath, walker, context);
+			break;
+		case T_GatherMerge:
+			path_tree_walker(((GatherMergePath *) path)->subpath, walker, context);
+			break;
+		default:
+			elog(ERROR, "unrecognized node type: %d", (int) path->pathtype);
+			break;
+	}
+}
+
+/*
+ * path_type_counter
+ *		Determine the total number of paths and the number of paths that are
+ *		parallel_aware and the number that are parallel safe.
+ */
+static void
+path_type_counter(Path *path, PathTypeCount *pathcount)
+{
+	pathcount->count++;
+	if (path->parallel_aware)
+		pathcount->parallel_aware_count++;
+	else if (path->parallel_safe)
+		pathcount->parallel_safe_count++;
+}
+
+/*
+ * contains_a_parallel_path
+ *		Determine if 'path' or any of its subpaths are parallel aware
+ */
+static bool
+contains_a_parallel_path(Path *path)
+{
+	PathTypeCount pathcount;
+
+	memset(&pathcount, 0, sizeof(pathcount));
+
+	path_tree_walker(path, path_type_counter, (void *) &pathcount);
+
+	return (pathcount.parallel_aware_count > 0);
+}
+
+/*
+ * contains_only_parallel_aware_and_safe_paths
+ *		Returns true if 'path' and all of its subpaths are either parallel
+ *		aware or parallel safe.
+ */
+static bool
+contains_only_parallel_aware_and_safe_paths(Path *path)
+{
+	PathTypeCount pathcount;
+
+	memset(&pathcount, 0, sizeof(pathcount));
+
+	path_tree_walker(path, path_type_counter, (void *) &pathcount);
+
+	return (pathcount.parallel_aware_count + pathcount.parallel_safe_count == pathcount.count);
+}
+
 /*
  * is_projection_capable_path
  *		Check whether a given Path node is able to do projection.