Assumptions about the number of parallel workers

diff --git a/src/backend/executor/nodeGather.c b/src/backend/executor/nodeGather.c
index 6b8ed867d5..09aecf7bfe 100644
--- a/src/backend/executor/nodeGather.c
+++ b/src/backend/executor/nodeGather.c
@@ -158,11 +158,17 @@ ExecGather(PlanState *pstate)
 		EState	   *estate = node->ps.state;
 		Gather	   *gather = (Gather *) node->ps.plan;
 
+		/*
+		 * Gather node should not be created if there are no parallel
+		 * workers.
+		 */
+		Assert(gather->num_workers > 0);
+
 		/*
 		 * Sometimes we might have to run without parallelism; but if parallel
 		 * mode is active then we can try to fire up some workers.
 		 */
-		if (gather->num_workers > 0 && estate->es_use_parallel_mode)
+		if (estate->es_use_parallel_mode)
 		{
 			ParallelContext *pcxt;
 
diff --git a/src/backend/executor/nodeGatherMerge.c b/src/backend/executor/nodeGatherMerge.c
index 317ddb4ae2..80c3f1561b 100644
--- a/src/backend/executor/nodeGatherMerge.c
+++ b/src/backend/executor/nodeGatherMerge.c
@@ -202,11 +202,17 @@ ExecGatherMerge(PlanState *pstate)
 		EState	   *estate = node->ps.state;
 		GatherMerge *gm = castNode(GatherMerge, node->ps.plan);
 
+		/*
+		 * GatherMerge node should not be created if there are no parallel
+		 * workers.
+		 */
+		Assert(gm->num_workers > 0);
+
 		/*
 		 * Sometimes we might have to run without parallelism; but if parallel
 		 * mode is active then we can try to fire up some workers.
 		 */
-		if (gm->num_workers > 0 && estate->es_use_parallel_mode)
+		if (estate->es_use_parallel_mode)
 		{
 			ParallelContext *pcxt;
 
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index d76fae44b8..a1fdd3afef 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -1907,7 +1907,6 @@ _outGatherPath(StringInfo str, const GatherPath *node)
 	_outPathInfo(str, (const Path *) node);
 
 	WRITE_NODE_FIELD(subpath);
-	WRITE_BOOL_FIELD(single_copy);
 	WRITE_INT_FIELD(num_workers);
 }
 
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index e048d200bb..ee3b8f7fd5 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -276,7 +276,7 @@ static Unique *make_unique_from_sortclauses(Plan *lefttree, List *distinctList);
 static Unique *make_unique_from_pathkeys(Plan *lefttree,
 										 List *pathkeys, int numCols);
 static Gather *make_gather(List *qptlist, List *qpqual,
-						   int nworkers, int rescan_param, bool single_copy, Plan *subplan);
+						   int nworkers, int rescan_param, Plan *subplan);
 static SetOp *make_setop(SetOpCmd cmd, SetOpStrategy strategy, Plan *lefttree,
 						 List *distinctList, AttrNumber flagColIdx, int firstFlag,
 						 long numGroups);
@@ -1727,7 +1727,6 @@ create_gather_plan(PlannerInfo *root, GatherPath *best_path)
 							  NIL,
 							  best_path->num_workers,
 							  assign_special_exec_param(root),
-							  best_path->single_copy,
 							  subplan);
 
 	copy_generic_path_info(&gather_plan->plan, &best_path->path);
@@ -6450,7 +6449,6 @@ make_gather(List *qptlist,
 			List *qpqual,
 			int nworkers,
 			int rescan_param,
-			bool single_copy,
 			Plan *subplan)
 {
 	Gather	   *node = makeNode(Gather);
@@ -6462,7 +6460,7 @@ make_gather(List *qptlist,
 	plan->righttree = NULL;
 	node->num_workers = nworkers;
 	node->rescan_param = rescan_param;
-	node->single_copy = single_copy;
+	node->single_copy = false;
 	node->invisible = false;
 	node->initParam = NULL;
 
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index e6d08aede5..31316cd2ec 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -758,6 +758,12 @@ add_partial_path(RelOptInfo *parent_rel, Path *new_path)
 	/* Path to be added must be parallel safe. */
 	Assert(new_path->parallel_safe);
 
+	/*
+	 * No partial path should be created if there's not enough work for
+	 * parallel workers.
+	 */
+	Assert(new_path->parallel_workers > 0);
+
 	/* Relation should be OK for parallelism, too. */
 	Assert(parent_rel->consider_parallel);
 
@@ -1847,6 +1853,7 @@ create_gather_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	GatherPath *pathnode = makeNode(GatherPath);
 
 	Assert(subpath->parallel_safe);
+	Assert(subpath->parallel_workers > 0);
 
 	pathnode->path.pathtype = T_Gather;
 	pathnode->path.parent = rel;
@@ -1860,14 +1867,6 @@ create_gather_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 
 	pathnode->subpath = subpath;
 	pathnode->num_workers = subpath->parallel_workers;
-	pathnode->single_copy = false;
-
-	if (pathnode->num_workers == 0)
-	{
-		pathnode->path.pathkeys = subpath->pathkeys;
-		pathnode->num_workers = 1;
-		pathnode->single_copy = true;
-	}
 
 	cost_gather(pathnode, root, rel, pathnode->path.param_info, rows);
 
diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h
index 3d3be197e0..3e81045b53 100644
--- a/src/include/nodes/pathnodes.h
+++ b/src/include/nodes/pathnodes.h
@@ -1458,14 +1458,12 @@ typedef struct UniquePath
 
 /*
  * GatherPath runs several copies of a plan in parallel and collects the
- * results.  The parallel leader may also execute the plan, unless the
- * single_copy flag is set.
+ * results.  The parallel leader may also execute the plan.
  */
 typedef struct GatherPath
 {
 	Path		path;
 	Path	   *subpath;		/* path for each worker */
-	bool		single_copy;	/* don't execute path more than once */
 	int			num_workers;	/* number of workers sought to help */
 } GatherPath;