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/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