Avoid making a separate pass over the query to check for partializability.

It's rather silly to make a separate pass over the tlist + HAVING qual,
and a separate set of visits to the syscache, when get_agg_clause_costs
already has all the required information in hand.  This nets out as less
code as well as fewer cycles.

src/include/nodes/relation.h

@@ -50,12 +50,15 @@ typedef struct QualCost
  * Costing aggregate function execution requires these statistics about
  * the aggregates to be executed by a given Agg node.  Note that the costs
  * include the execution costs of the aggregates' argument expressions as
- * well as the aggregate functions themselves.
+ * well as the aggregate functions themselves.  Also, the fields must be
+ * defined so that initializing the struct to zeroes with memset is correct.
  */
 typedef struct AggClauseCosts
 {
 	int			numAggs;		/* total number of aggregate functions */
 	int			numOrderedAggs; /* number w/ DISTINCT/ORDER BY/WITHIN GROUP */
+	bool		hasNonPartial;	/* does any agg not support partial mode? */
+	bool		hasNonSerial;	/* is any partial agg non-serializable? */
 	QualCost	transCost;		/* total per-input-row execution costs */
 	Cost		finalCost;		/* total per-aggregated-row costs */
 	Size		transitionSpace;	/* space for pass-by-ref transition data */