v50-0003-comment.patch

From 1805c68b0ae14382952c49b77ccbe30cd7605baa Mon Sep 17 00:00:00 2001
From: James Coleman <jtc331@gmail.com>
Date: Wed, 1 Apr 2020 09:04:39 -0400
Subject: [PATCH v50 3/6] comment

---
 src/backend/optimizer/plan/planner.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index cc4718d1c9..aeb83841d7 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -4986,6 +4986,11 @@ create_ordered_paths(PlannerInfo *root,
 		}
 		else
 		{
+			/*
+			 * Try adding an explicit sort, but only to the cheapest total path
+			 * since a full sort should generally add the same cost to all
+			 * paths.
+			 */
 			if (input_path == cheapest_input_path)
 			{
 				/*
@@ -5005,7 +5010,13 @@ create_ordered_paths(PlannerInfo *root,
 				add_path(ordered_rel, sorted_path);
 			}
 
-			/* With incremental sort disabled, don't build those paths. */
+			/*
+			 * If incremental sort is enabled, then try it as well. Unlike with
+			 * regular sorts, we can't just look at the cheapest path, because
+			 * the cost of incremental sort depends on how well presorted the
+			 * path is. Additionally incremental sort may enable a cheaper
+			 * startup path to win out despite higher total cost.
+			 */
 			if (!enable_incrementalsort)
 				continue;
 
-- 
2.17.1