v49-0004-ignore-single-key-orderings.patch

From c0c82add7a9cf58584683abe5d498156424beebd Mon Sep 17 00:00:00 2001
From: James Coleman <jtc331@gmail.com>
Date: Tue, 31 Mar 2020 20:32:08 -0400
Subject: [PATCH v49 4/7] ignore single key orderings

---
 src/backend/optimizer/path/allpaths.c |  7 +++++--
 src/backend/optimizer/plan/planner.c  | 14 +++++++-------
 2 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 93d967e812..b332e474b8 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -2842,7 +2842,10 @@ generate_useful_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_r
 			Path	   *subpath = (Path *) lfirst(lc2);
 			GatherMergePath *path;
 
-			/* path has no ordering at all, can't use incremental sort */
+			/*
+			 * If the path has no ordering at all, then we can't use either
+			 * incremental sort or rely on implict sorting with a gather merge.
+			 */
 			if (subpath->pathkeys == NIL)
 				continue;
 
@@ -2907,7 +2910,7 @@ generate_useful_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_r
 			 * Consider incremental sort, but only when the subpath is already
 			 * partially sorted on a pathkey prefix.
 			 */
-			if (enable_incrementalsort && presorted_keys > 0)
+			if (enable_incrementalsort && presorted_keys > 0 && list_length(useful_pathkeys) > 1)
 			{
 				Path	   *tmp;
 
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 73b7782dcb..6592d0446d 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -5010,7 +5010,7 @@ create_ordered_paths(PlannerInfo *root,
 				continue;
 
 			/* Likewise, if the path can't be used for incremental sort. */
-			if (!presorted_keys)
+			if (!presorted_keys || list_length(root->sort_pathkeys) == 1)
 				continue;
 
 			/* Also consider incremental sort. */
@@ -5086,7 +5086,7 @@ create_ordered_paths(PlannerInfo *root,
 		 * XXX This is probably duplicate with the paths we already generate
 		 * in generate_useful_gather_paths in apply_scanjoin_target_to_paths.
 		 */
-		if (enable_incrementalsort)
+		if (enable_incrementalsort && list_length(root->sort_pathkeys) > 1)
 		{
 			ListCell   *lc;
 
@@ -6561,7 +6561,7 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 			 * when the path is not already sorted and when incremental sort
 			 * is enabled.
 			 */
-			if (is_sorted || !enable_incrementalsort)
+			if (is_sorted || !enable_incrementalsort || list_length(root->group_pathkeys) == 1)
 				continue;
 
 			/* Restore the input path (we might have added Sort on top). */
@@ -6688,7 +6688,7 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 				 * when the path is not already sorted and when incremental
 				 * sort is enabled.
 				 */
-				if (is_sorted || !enable_incrementalsort)
+				if (is_sorted || !enable_incrementalsort || list_length(root->group_pathkeys) == 1)
 					continue;
 
 				/* Restore the input path (we might have added Sort on top). */
@@ -7005,7 +7005,7 @@ create_partial_grouping_paths(PlannerInfo *root,
 		}
 
 		/* Consider incremental sort on all partial paths, if enabled. */
-		if (enable_incrementalsort)
+		if (enable_incrementalsort && list_length(root->group_pathkeys) > 1)
 		{
 			foreach(lc, input_rel->pathlist)
 			{
@@ -7106,7 +7106,7 @@ create_partial_grouping_paths(PlannerInfo *root,
 			 * when the path is not already sorted and when incremental sort
 			 * is enabled.
 			 */
-			if (is_sorted || !enable_incrementalsort)
+			if (is_sorted || !enable_incrementalsort || list_length(root->group_pathkeys) == 1)
 				continue;
 
 			/* Restore the input path (we might have added Sort on top). */
@@ -7278,7 +7278,7 @@ gather_grouping_paths(PlannerInfo *root, RelOptInfo *rel)
 		add_path(rel, path);
 	}
 
-	if (!enable_incrementalsort)
+	if (!enable_incrementalsort || list_length(root->group_pathkeys) == 1)
 		return;
 
 	/* also consider incremental sort on partial paths, if enabled */
-- 
2.17.1