v01_enable_limit_adjust_cost_guc.patch

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 60b0fcfb6be..8e8be605aed 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -163,7 +163,7 @@ bool		enable_parallel_hash = true;
 bool		enable_partition_pruning = true;
 bool		enable_presorted_aggregate = true;
 bool		enable_async_append = true;
-
+bool 		enable_limit_adjust_cost = true;
 typedef struct
 {
 	PlannerInfo *root;
diff --git a/src/backend/optimizer/plan/planagg.c b/src/backend/optimizer/plan/planagg.c
index 64605be3178..510941feac0 100644
--- a/src/backend/optimizer/plan/planagg.c
+++ b/src/backend/optimizer/plan/planagg.c
@@ -412,12 +412,20 @@ build_minmax_path(PlannerInfo *root, MinMaxAggInfo *mminfo,
 										   Int64GetDatum(1), false,
 										   FLOAT8PASSBYVAL);
 
-	/*
-	 * Generate the best paths for this query, telling query_planner that we
-	 * have LIMIT 1.
-	 */
-	subroot->tuple_fraction = 1.0;
-	subroot->limit_tuples = 1.0;
+	if (enable_limit_adjust_cost)
+	{
+		/*
+		 * Generate the best paths for this query, telling query_planner that
+		 * we have LIMIT 1.
+		 */
+		subroot->tuple_fraction = 1.0;
+		subroot->limit_tuples = 1.0;
+	}
+	else
+	{
+		subroot->tuple_fraction = 0.0;
+		subroot->limit_tuples = -1.0;
+	}
 
 	final_rel = query_planner(subroot, minmax_qp_callback, NULL);
 
@@ -432,9 +440,11 @@ build_minmax_path(PlannerInfo *root, MinMaxAggInfo *mminfo,
 
 	/*
 	 * Get the best presorted path, that being the one that's cheapest for
-	 * fetching just one row.  If there's no such path, fail.
+	 * fetching just one row.  If there's no such path, fail.  If the
+	 * adjustance of path cost is disabled, we will also set path_fraction
+	 * to 1.0.
 	 */
-	if (final_rel->rows > 1.0)
+	if (enable_limit_adjust_cost && final_rel->rows > 1.0)
 		path_fraction = 1.0 / final_rel->rows;
 	else
 		path_fraction = 1.0;
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 566ce5b3cb4..bb6fc47be61 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -1401,12 +1401,25 @@ grouping_planner(PlannerInfo *root, double tuple_fraction,
 		tuple_fraction = preprocess_limit(root, tuple_fraction,
 										  &offset_est, &count_est);
 
-		/*
-		 * If we have a known LIMIT, and don't have an unknown OFFSET, we can
-		 * estimate the effects of using a bounded sort.
-		 */
-		if (count_est > 0 && offset_est >= 0)
-			limit_tuples = (double) count_est + (double) offset_est;
+		if (enable_limit_adjust_cost)
+		{
+			/*
+			 * If we have a known LIMIT, and don't have an unknown OFFSET, we
+			 * can estimate the effects of using a bounded sort.
+			 */
+			if (count_est > 0 && offset_est >= 0)
+				limit_tuples = (double) count_est + (double) offset_est;
+		}
+		else
+		{
+			/*
+			 * Disable limit clause adjust cost, retrival all tuples.
+			 * offset_est and count_est are needed to adjust estimated rows of
+			 * limit path. Thus, we should do this after calling
+			 * preprocess_limit().
+			 */
+			tuple_fraction = 0.0;
+		}
 	}
 
 	/* Make tuple_fraction accessible to lower-level routines */
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 93e73cb44db..2f8465d0435 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -4079,6 +4079,13 @@ adjust_limit_rows_costs(double *rows,	/* in/out parameter */
 		if (*rows < 1)
 			*rows = 1;
 	}
+
+	/* disable cost modification */
+	if (!enable_limit_adjust_cost)
+	{
+		*startup_cost = input_startup_cost;
+		*total_cost = input_total_cost;
+	}
 }
 
 
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 60b12446a1c..3a46ca17a80 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -1036,6 +1036,16 @@ struct config_bool ConfigureNamesBool[] =
 		true,
 		NULL, NULL, NULL
 	},
+	{
+		{"enable_limit_adjust_cost", PGC_USERSET, QUERY_TUNING_METHOD,
+			gettext_noop("Enable the planner's use of limit information adjust cost."),
+			NULL,
+			GUC_EXPLAIN
+		},
+		&enable_limit_adjust_cost,
+		true,
+		NULL, NULL, NULL
+	},
 	{
 		{"geqo", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("Enables genetic query optimization."),
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h
index c5987440817..7cdac2ec0e6 100644
--- a/src/include/optimizer/cost.h
+++ b/src/include/optimizer/cost.h
@@ -71,6 +71,7 @@ extern PGDLLIMPORT bool enable_partition_pruning;
 extern PGDLLIMPORT bool enable_presorted_aggregate;
 extern PGDLLIMPORT bool enable_async_append;
 extern PGDLLIMPORT int constraint_exclusion;
+extern PGDLLIMPORT bool enable_limit_adjust_cost;
 
 extern double index_pages_fetched(double tuples_fetched, BlockNumber pages,
 								  double index_pages, PlannerInfo *root);