Add parallel_leader_participation GUC.

Sometimes, for testing, it's useful to have the leader do nothing but
read tuples from workers; and it's possible that could work out better
even in production.

Thomas Munro, reviewed by Amit Kapila and by me.  A few final tweaks
by me.

Discussion: http://postgr.es/m/CAEepm=2U++Lp3bNTv2Bv_kkr5NE2pOyHhxU=G0YTa4ZhSYhHiw@mail.gmail.com

src/backend/executor/nodeGather.c

@@ -38,6 +38,7 @@
 #include "executor/nodeSubplan.h"
 #include "executor/tqueue.h"
 #include "miscadmin.h"
+#include "optimizer/planmain.h"
 #include "pgstat.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
@@ -73,7 +74,8 @@ ExecInitGather(Gather *node, EState *estate, int eflags)
 	gatherstate->ps.ExecProcNode = ExecGather;
 
 	gatherstate->initialized = false;
-	gatherstate->need_to_scan_locally = !node->single_copy;
+	gatherstate->need_to_scan_locally =
+		!node->single_copy && parallel_leader_participation;
 	gatherstate->tuples_needed = -1;
 
 	/*
@@ -193,9 +195,9 @@ ExecGather(PlanState *pstate)
 			node->nextreader = 0;
 		}
 
-		/* Run plan locally if no workers or not single-copy. */
+		/* Run plan locally if no workers or enabled and not single-copy. */
 		node->need_to_scan_locally = (node->nreaders == 0)
-			|| !gather->single_copy;
+			|| (!gather->single_copy && parallel_leader_participation);
 		node->initialized = true;
 	}
 

src/backend/executor/nodeGatherMerge.c

@@ -23,6 +23,7 @@
 #include "executor/tqueue.h"
 #include "lib/binaryheap.h"
 #include "miscadmin.h"
+#include "optimizer/planmain.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
 
@@ -233,8 +234,9 @@ ExecGatherMerge(PlanState *pstate)
 			}
 		}
 
-		/* always allow leader to participate */
-		node->need_to_scan_locally = true;
+		/* allow leader to participate if enabled or no choice */
+		if (parallel_leader_participation || node->nreaders == 0)
+			node->need_to_scan_locally = true;
 		node->initialized = true;
 	}
 

src/backend/optimizer/path/costsize.c

@@ -5137,7 +5137,6 @@ static double
 get_parallel_divisor(Path *path)
 {
 	double		parallel_divisor = path->parallel_workers;
-	double		leader_contribution;
 
 	/*
 	 * Early experience with parallel query suggests that when there is only
@@ -5150,9 +5149,14 @@ get_parallel_divisor(Path *path)
 	 * its time servicing each worker, and the remainder executing the
 	 * parallel plan.
 	 */
-	leader_contribution = 1.0 - (0.3 * path->parallel_workers);
-	if (leader_contribution > 0)
-		parallel_divisor += leader_contribution;
+	if (parallel_leader_participation)
+	{
+		double		leader_contribution;
+
+		leader_contribution = 1.0 - (0.3 * path->parallel_workers);
+		if (leader_contribution > 0)
+			parallel_divisor += leader_contribution;
+	}
 
 	return parallel_divisor;
 }

src/backend/optimizer/plan/planner.c

@@ -61,6 +61,7 @@
 /* GUC parameters */
 double		cursor_tuple_fraction = DEFAULT_CURSOR_TUPLE_FRACTION;
 int			force_parallel_mode = FORCE_PARALLEL_OFF;
+bool		parallel_leader_participation = true;
 
 /* Hook for plugins to get control in planner() */
 planner_hook_type planner_hook = NULL;

src/backend/utils/misc/guc.c

@@ -1676,6 +1676,16 @@ static struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"parallel_leader_participation", PGC_USERSET, RESOURCES_ASYNCHRONOUS,
+			gettext_noop("Controls whether Gather and Gather Merge also run subplans."),
+			gettext_noop("Should gather nodes also run subplans, or just gather tuples?")
+		},
+		&parallel_leader_participation,
+		true,
+		NULL, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, false, NULL, NULL, NULL

src/backend/utils/misc/postgresql.conf.sample

@@ -163,6 +163,7 @@
 #effective_io_concurrency = 1		# 1-1000; 0 disables prefetching
 #max_worker_processes = 8		# (change requires restart)
 #max_parallel_workers_per_gather = 2	# taken from max_parallel_workers
+#parallel_leader_particulation = on
 #max_parallel_workers = 8		# maximum number of max_worker_processes that
 					# can be used in parallel queries
 #old_snapshot_threshold = -1		# 1min-60d; -1 disables; 0 is immediate