Redesign get_attstatsslot()/free_attstatsslot() for more safety and speed.

The mess cleaned up in commit da0759600 is clear evidence that it's a
bug hazard to expect the caller of get_attstatsslot()/free_attstatsslot()
to provide the correct type OID for the array elements in the slot.
Moreover, we weren't even getting any performance benefit from that,
since get_attstatsslot() was extracting the real type OID from the array
anyway.  So we ought to get rid of that requirement; indeed, it would
make more sense for get_attstatsslot() to pass back the type OID it found,
in case the caller isn't sure what to expect, which is likely in binary-
compatible-operator cases.

Another problem with the current implementation is that if the stats array
element type is pass-by-reference, we incur a palloc/memcpy/pfree cycle
for each element.  That seemed acceptable when the code was written because
we were targeting O(10) array sizes --- but these days, stats arrays are
almost always bigger than that, sometimes much bigger.  We can save a
significant number of cycles by doing one palloc/memcpy/pfree of the whole
array.  Indeed, in the now-probably-common case where the array is toasted,
that happens anyway so this method is basically free.  (Note: although the
catcache code will inline any out-of-line toasted values, it doesn't
decompress them.  At the other end of the size range, it doesn't expand
short-header datums either.  In either case, DatumGetArrayTypeP would have
to make a copy.  We do end up using an extra array copy step if the element
type is pass-by-value and the array length is neither small enough for a
short header nor large enough to have suffered compression.  But that
seems like a very acceptable price for winning in pass-by-ref cases.)

Hence, redesign to take these insights into account.  While at it,
convert to an API in which we fill a struct rather than passing a bunch
of pointers to individual output arguments.  That will make it less
painful if we ever want further expansion of what get_attstatsslot can
pass back.

It's certainly arguable that this is new development and not something to
push post-feature-freeze.  However, I view it as primarily bug-proofing
and therefore something that's better to have sooner not later.  Since
we aren't quite at beta phase yet, let's put it in.

Discussion: https://postgr.es/m/16364.1494520862@sss.pgh.pa.us

src/include/utils/lsyscache.h

@@ -35,6 +35,28 @@ typedef enum IOFuncSelector
 	IOFunc_send
 } IOFuncSelector;
 
+/* Flag bits for get_attstatsslot */
+#define ATTSTATSSLOT_VALUES		0x01
+#define ATTSTATSSLOT_NUMBERS	0x02
+
+/* Result struct for get_attstatsslot */
+typedef struct AttStatsSlot
+{
+	/* Always filled: */
+	Oid			staop;			/* Actual staop for the found slot */
+	/* Filled if ATTSTATSSLOT_VALUES is specified: */
+	Oid			valuetype;		/* Actual datatype of the values */
+	Datum	   *values;			/* slot's "values" array, or NULL if none */
+	int			nvalues;		/* length of values[], or 0 */
+	/* Filled if ATTSTATSSLOT_NUMBERS is specified: */
+	float4	   *numbers;		/* slot's "numbers" array, or NULL if none */
+	int			nnumbers;		/* length of numbers[], or 0 */
+
+	/* Remaining fields are private to get_attstatsslot/free_attstatsslot */
+	void	   *values_arr;		/* palloc'd values array, if any */
+	void	   *numbers_arr;	/* palloc'd numbers array, if any */
+} AttStatsSlot;
+
 /* Hook for plugins to get control in get_attavgwidth() */
 typedef int32 (*get_attavgwidth_hook_type) (Oid relid, AttrNumber attnum);
 extern PGDLLIMPORT get_attavgwidth_hook_type get_attavgwidth_hook;
@@ -148,15 +170,9 @@ extern Oid	getBaseType(Oid typid);
 extern Oid	getBaseTypeAndTypmod(Oid typid, int32 *typmod);
 extern int32 get_typavgwidth(Oid typid, int32 typmod);
 extern int32 get_attavgwidth(Oid relid, AttrNumber attnum);
-extern bool get_attstatsslot(HeapTuple statstuple,
-				 Oid atttype, int32 atttypmod,
-				 int reqkind, Oid reqop,
-				 Oid *actualop,
-				 Datum **values, int *nvalues,
-				 float4 **numbers, int *nnumbers);
-extern void free_attstatsslot(Oid atttype,
-				  Datum *values, int nvalues,
-				  float4 *numbers, int nnumbers);
+extern bool get_attstatsslot(AttStatsSlot *sslot, HeapTuple statstuple,
+				 int reqkind, Oid reqop, int flags);
+extern void free_attstatsslot(AttStatsSlot *sslot);
 extern char *get_namespace_name(Oid nspid);
 extern char *get_namespace_name_or_temp(Oid nspid);
 extern Oid	get_range_subtype(Oid rangeOid);