Support MergeAppend plans, to allow sorted output from append relations.

This patch eliminates the former need to sort the output of an Append scan
when an ordered scan of an inheritance tree is wanted.  This should be
particularly useful for fast-start cases such as queries with LIMIT.

Original patch by Greg Stark, with further hacking by Hans-Jurgen Schonig,
Robert Haas, and Tom Lane.

src/include/nodes/execnodes.h

@@ -1049,6 +1049,33 @@ typedef struct AppendState
 	int			as_whichplan;
 } AppendState;
 
+/* ----------------
+ *	 MergeAppendState information
+ *
+ *		nplans			how many plans are in the array
+ *		nkeys			number of sort key columns
+ *		scankeys		sort keys in ScanKey representation
+ *		slots			current output tuple of each subplan
+ *		heap			heap of active tuples (represented as array indexes)
+ *		heap_size		number of active heap entries
+ *		initialized		true if we have fetched first tuple from each subplan
+ *		last_slot		last subplan fetched from (which must be re-called)
+ * ----------------
+ */
+typedef struct MergeAppendState
+{
+	PlanState	ps;				/* its first field is NodeTag */
+	PlanState **mergeplans;		/* array of PlanStates for my inputs */
+	int			ms_nplans;
+	int			ms_nkeys;
+	ScanKey		ms_scankeys;	/* array of length ms_nkeys */
+	TupleTableSlot **ms_slots;	/* array of length ms_nplans */
+	int			*ms_heap;		/* array of length ms_nplans */
+	int			ms_heap_size;	/* current active length of ms_heap[] */
+	bool		ms_initialized;	/* are subplans started? */
+	int			ms_last_slot;	/* last subplan slot we returned from */
+} MergeAppendState;
+
 /* ----------------
  *	 RecursiveUnionState information
  *