Fix cost estimates for EXISTS subqueries that are evaluated as initPlans

(because they are uncorrelated with the immediate parent query).  We were
charging the full run cost to the parent node, disregarding the fact that
only one row need be fetched for EXISTS.  While this would only be a
cosmetic issue in most cases, it might possibly affect planning outcomes
if the parent query were itself a subquery to some upper query.
Per recent discussion with Steve Crawford.

src/backend/optimizer/path/costsize.c

@@ -54,7 +54,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.185 2007/06/11 01:16:22 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.186 2007/09/22 21:36:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2049,9 +2049,10 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
 		{
 			/*
 			 * Otherwise we will be rescanning the subplan output on each
-			 * evaluation.	We need to estimate how much of the output we will
-			 * actually need to scan.  NOTE: this logic should agree with the
-			 * estimates used by make_subplan() in plan/subselect.c.
+			 * evaluation.  We need to estimate how much of the output we will
+			 * actually need to scan.  NOTE: this logic should agree with
+			 * get_initplan_cost, below, and with the estimates used by
+			 * make_subplan() in plan/subselect.c.
 			 */
 			Cost		plan_run_cost = plan->total_cost - plan->startup_cost;
 
@@ -2097,6 +2098,43 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
 }
 
 
+/*
+ * get_initplan_cost
+ *		Get the expected cost of evaluating an initPlan.
+ *
+ * Keep this in sync with cost_qual_eval_walker's handling of subplans, above,
+ * and with the estimates used by make_subplan() in plan/subselect.c.
+ */
+Cost
+get_initplan_cost(PlannerInfo *root, SubPlan *subplan)
+{
+	Cost		result;
+	Plan	   *plan = planner_subplan_get_plan(root, subplan);
+
+	/* initPlans never use hashtables */
+	Assert(!subplan->useHashTable);
+	/* they are never ALL or ANY, either */
+	Assert(!(subplan->subLinkType == ALL_SUBLINK ||
+			 subplan->subLinkType == ANY_SUBLINK));
+
+	if (subplan->subLinkType == EXISTS_SUBLINK)
+	{
+		/* we only need to fetch 1 tuple */
+		Cost		plan_run_cost = plan->total_cost - plan->startup_cost;
+
+		result = plan->startup_cost;
+		result += plan_run_cost / plan->plan_rows;
+	}
+	else
+	{
+		/* assume we need all tuples */
+		result = plan->total_cost;
+	}
+
+	return result;
+}
+
+
 /*
  * approx_selectivity
  *		Quick-and-dirty estimation of clause selectivities.