From: Robert Haas <rhaas@postgresql.org>
Date: Thu, 19 Feb 2026 18:46:10 +0000 (-0500)
Subject: Fix add_partial_path interaction with disabled_nodes
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=6e466e1e839e2d40b30af9cbda107af867d259f6;p=thirdparty%2Fpostgresql.git

Fix add_partial_path interaction with disabled_nodes

Commit e22253467942fdb100087787c3e1e3a8620c54b2 adjusted the logic in
add_path() to keep the path list sorted by disabled_nodes and then by
total_cost, but failed to make the corresponding adjustment to
add_partial_path. As a result, add_partial_path might sort the path list
just by total cost, which could lead to later planner misbehavior.

In principle, this should be back-patched to v18, but we are typically
reluctant to back-patch planner fixes for fear of destabilizing working
installations, and it is unclear to me that this has sufficiently
serious consequences to justify an exception, so for now, no back-patch.

Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Discussion: http://postgr.es/m/CAMbWs4-mO3jMK4t_LgcJ+7Eo=NmGgkxettgRaVbJzZvVZ1koMA@mail.gmail.com
---

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 9678c20ff1f..ef8ef6e89d3 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -758,9 +758,10 @@ add_path_precheck(RelOptInfo *parent_rel, int disabled_nodes,
  *	  parallel such that each worker will generate a subset of the path's
  *	  overall result.
  *
- *	  As in add_path, the partial_pathlist is kept sorted with the cheapest
- *	  total path in front.  This is depended on by multiple places, which
- *	  just take the front entry as the cheapest path without searching.
+ *	  As in add_path, the partial_pathlist is kept sorted first by smallest
+ *    number of disabled nodes and then by lowest total cost. This is depended
+ *    on by multiple places, which just take the front entry as the cheapest
+ *    path without searching.
  *
  *	  We don't generate parameterized partial paths for several reasons.  Most
  *	  importantly, they're not safe to execute, because there's nothing to
@@ -878,8 +879,13 @@ add_partial_path(RelOptInfo *parent_rel, Path *new_path)
 		}
 		else
 		{
-			/* new belongs after this old path if it has cost >= old's */
-			if (new_path->total_cost >= old_path->total_cost)
+			/*
+			 * new belongs after this old path if it has more disabled nodes
+			 * or if it has the same number of nodes but a greater total cost
+			 */
+			if (new_path->disabled_nodes > old_path->disabled_nodes ||
+				(new_path->disabled_nodes == old_path->disabled_nodes &&
+				 new_path->total_cost >= old_path->total_cost))
 				insert_at = foreach_current_index(p1) + 1;
 		}