commit 02b738b5e32326a24687890e066c66f5508b5976
Author: Tomas Vondra <tomas@2ndquadrant.com>
Date:   Sun Jul 28 15:55:54 2019 +0200

    Consider low startup cost when adding partial path
    
    45be99f8cd5d606086e0a458c9c72910ba8a613d added `add_partial_path` with the
    comment:
    
    > Neither do we need to consider startup costs:
    > parallelism is only used for plans that will be run to completion.
    > Therefore, this routine is much simpler than add_path: it needs to
    > consider only pathkeys and total cost.
    
    I'm not entirely sure if that is still true or not--I can't easily come
    up with a scenario in which it's not, but I also can't come up with an
    inherent reason why such a scenario cannot exist.
    
    Regardless, the in-progress incremental sort patch uncovered a new case
    where it definitely no longer holds, and, as a result a higher cost plan
    ends up being chosen because a low startup cost partial path is ignored
    in favor of a lower total cost partial path and a limit is a applied on
    top of that which would normal favor the lower startup cost plan.

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 34acb732ee..5d66fc2177 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -778,41 +778,30 @@ add_partial_path(RelOptInfo *parent_rel, Path *new_path)
 		/* Unless pathkeys are incompatible, keep just one of the two paths. */
 		if (keyscmp != PATHKEYS_DIFFERENT)
 		{
-			if (new_path->total_cost > old_path->total_cost * STD_FUZZ_FACTOR)
-			{
-				/* New path costs more; keep it only if pathkeys are better. */
-				if (keyscmp != PATHKEYS_BETTER1)
-					accept_new = false;
-			}
-			else if (old_path->total_cost > new_path->total_cost
-					 * STD_FUZZ_FACTOR)
+			PathCostComparison costcmp;
+
+			/*
+			 * Do a fuzzy cost comparison with standard fuzziness limit.
+			 */
+			costcmp = compare_path_costs_fuzzily(new_path, old_path,
+												 STD_FUZZ_FACTOR);
+
+			if (costcmp == COSTS_BETTER1)
 			{
-				/* Old path costs more; keep it only if pathkeys are better. */
-				if (keyscmp != PATHKEYS_BETTER2)
+				if (keyscmp == PATHKEYS_BETTER1)
 					remove_old = true;
 			}
-			else if (keyscmp == PATHKEYS_BETTER1)
-			{
-				/* Costs are about the same, new path has better pathkeys. */
-				remove_old = true;
-			}
-			else if (keyscmp == PATHKEYS_BETTER2)
+			else if (costcmp == COSTS_BETTER2)
 			{
-				/* Costs are about the same, old path has better pathkeys. */
-				accept_new = false;
-			}
-			else if (old_path->total_cost > new_path->total_cost * 1.0000000001)
-			{
-				/* Pathkeys are the same, and the old path costs more. */
-				remove_old = true;
+				if (keyscmp == PATHKEYS_BETTER2)
+					accept_new = false;
 			}
-			else
+			else if (costcmp == COSTS_EQUAL)
 			{
-				/*
-				 * Pathkeys are the same, and new path isn't materially
-				 * cheaper.
-				 */
-				accept_new = false;
+				if (keyscmp == PATHKEYS_BETTER1)
+					remove_old = true;
+				else if (keyscmp == PATHKEYS_BETTER2)
+					accept_new = false;
 			}
 		}