Respect disabled_nodes in fix_alternative_subplan.

When my commit e222534679 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.

As usual for planner fixes that might destabilize plan choices,
no back-patch.

Discussion: https://postgr.es/m/CA+TgmoaK=4w7-qknUo3QhUJ53pXZq=c=KgZmRyD+k7ytqfmgSg@mail.gmail.com
Reviewed-by: Lukas Fittl <lukas@fittl.com>

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;
 }

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;

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;