Tweak EXPLAIN for parallel query to show workers launched.

The previous display was sort of confusing, because it didn't
distinguish between the number of workers that we planned to launch
and the number that actually got launched.  This has already confused
several people, so display both numbers and label them clearly.

Julien Rouhaud, reviewed by me.

src/backend/commands/explain.c

@@ -1339,8 +1339,16 @@ ExplainNode(PlanState *planstate, List *ancestors,
 				if (plan->qual)
 					show_instrumentation_count("Rows Removed by Filter", 1,
 											   planstate, es);
-				ExplainPropertyInteger("Number of Workers",
+				ExplainPropertyInteger("Workers Planned",
 									   gather->num_workers, es);
+				if (es->analyze)
+				{
+					int			nworkers;
+
+					nworkers = ((GatherState *) planstate)->nworkers_launched;
+					ExplainPropertyInteger("Workers Launched",
+										   nworkers, es);
+				}
 				if (gather->single_copy)
 					ExplainPropertyText("Single Copy",
 									  gather->single_copy ? "true" : "false",

src/backend/executor/nodeGather.c

@@ -166,6 +166,7 @@ ExecGather(GatherState *node)
 			 */
 			pcxt = node->pei->pcxt;
 			LaunchParallelWorkers(pcxt);
+			node->nworkers_launched = pcxt->nworkers_launched;
 
 			/* Set up tuple queue readers to read the results. */
 			if (pcxt->nworkers_launched > 0)

src/include/nodes/execnodes.h

@@ -1956,6 +1956,7 @@ typedef struct GatherState
 	struct ParallelExecutorInfo *pei;
 	int			nreaders;
 	int			nextreader;
+	int			nworkers_launched;
 	struct TupleQueueReader **reader;
 	TupleTableSlot *funnel_slot;
 	bool		need_to_scan_locally;