v22-0001-Respect-disabled_nodes-in-fix_alternative_subpla.patch

From e892484828a09ce6ca6f8943c7b3e650a53050ce Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Fri, 20 Mar 2026 14:04:41 -0400
Subject: [PATCH v22 1/6] Respect disabled_nodes in fix_alternative_subplan.

When my commit 12444183e40187a9fb6002a3912053f302725f0a added the
concept of disabled_nodes, it failed to add a disabled_nodes field
to SubPlan. This is a regression: before that commit, when
fix_alternative_subplan compared the costs of two plans, the number
of disabled nodes affected the result, because it was just a
component of the total cost. After that commit, it no longer did,
making it possible for a disabled path to win on cost over one that
is not disabled. Fix that.
---
 src/backend/optimizer/path/costsize.c |  1 +
 src/backend/optimizer/plan/setrefs.c  | 14 ++++++++++----
 src/include/nodes/primnodes.h         |  1 +
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 56d45287c89..1c575e56ff6 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -4761,6 +4761,7 @@ cost_subplan(PlannerInfo *root, SubPlan *subplan, Plan *plan)
 			sp_cost.per_tuple += plan->startup_cost;
 	}
 
+	subplan->disabled_nodes = plan->disabled_nodes;
 	subplan->startup_cost = sp_cost.startup;
 	subplan->per_call_cost = sp_cost.per_tuple;
 }
diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index 1b5b9b5ed9c..ff0e875f2a2 100644
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/src/backend/optimizer/plan/setrefs.c
@@ -2234,9 +2234,12 @@ fix_alternative_subplan(PlannerInfo *root, AlternativeSubPlan *asplan,
 
 	/*
 	 * Compute the estimated cost of each subplan assuming num_exec
-	 * executions, and keep the cheapest one.  In event of exact equality of
-	 * estimates, we prefer the later plan; this is a bit arbitrary, but in
-	 * current usage it biases us to break ties against fast-start subplans.
+	 * executions, and keep the cheapest one.  If one subplan has more
+	 * disabled nodes than another, choose the one with fewer disabled nodes
+	 * regardless of cost; this parallels compare_path_costs.  In event of
+	 * exact equality of estimates, we prefer the later plan; this is a bit
+	 * arbitrary, but in current usage it biases us to break ties against
+	 * fast-start subplans.
 	 */
 	Assert(asplan->subplans != NIL);
 
@@ -2246,7 +2249,10 @@ fix_alternative_subplan(PlannerInfo *root, AlternativeSubPlan *asplan,
 		Cost		curcost;
 
 		curcost = curplan->startup_cost + num_exec * curplan->per_call_cost;
-		if (bestplan == NULL || curcost <= bestcost)
+		if (bestplan == NULL ||
+			curplan->disabled_nodes < bestplan->disabled_nodes ||
+			(curplan->disabled_nodes == bestplan->disabled_nodes &&
+			 curcost <= bestcost))
 		{
 			bestplan = curplan;
 			bestcost = curcost;
diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index b67e56e6c5a..f5b6b45664a 100644
--- a/src/include/nodes/primnodes.h
+++ b/src/include/nodes/primnodes.h
@@ -1124,6 +1124,7 @@ typedef struct SubPlan
 	List	   *parParam;		/* indices of input Params from parent plan */
 	List	   *args;			/* exprs to pass as parParam values */
 	/* Estimated execution costs: */
+	int			disabled_nodes; /* count of disabled nodes in the plan */
 	Cost		startup_cost;	/* one-time setup cost */
 	Cost		per_call_cost;	/* cost for each subplan evaluation */
 } SubPlan;
-- 
2.51.0