Invent min_parallel_relation_size GUC to replace a hard-wired constant.

The main point of doing this is to allow the cutoff to be set very small,
even zero, to allow parallel-query behavior to be tested on relatively
small tables such as we typically use in the regression tests.  But it
might be of use to users too.  The number-of-workers scaling behavior in
create_plain_partial_paths() is pretty ad-hoc and subject to change, so
we won't expose anything about that, but the notion of not considering
parallel query at all for tables below size X seems reasonably stable.

Amit Kapila, per a suggestion from me

Discussion: <17170.1465830165@sss.pgh.pa.us>

src/backend/utils/misc/guc.c

@@ -2747,6 +2747,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"min_parallel_relation_size", PGC_USERSET, QUERY_TUNING_COST,
+			gettext_noop("Sets the minimum size of relations to be considered for parallel scan."),
+			NULL,
+			GUC_UNIT_BLOCKS,
+		},
+		&min_parallel_relation_size,
+		1024, 0, INT_MAX / 3,
+		NULL, NULL, NULL
+	},
+
 	{
 		/* Can't be set in postgresql.conf */
 		{"server_version_num", PGC_INTERNAL, PRESET_OPTIONS,

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

@@ -304,6 +304,7 @@
 #cpu_operator_cost = 0.0025		# same scale as above
 #parallel_tuple_cost = 0.1		# same scale as above
 #parallel_setup_cost = 1000.0	# same scale as above
+#min_parallel_relation_size = 8MB
 #effective_cache_size = 4GB
 
 # - Genetic Query Optimizer -