Add SQL-accessible functions for inspecting index AM properties.

Per discussion, we should provide such functions to replace the lost
ability to discover AM properties by inspecting pg_am (cf commit
65c5fcd35).  The added functionality is also meant to displace any code
that was looking directly at pg_index.indoption, since we'd rather not
believe that the bit meanings in that field are part of any client API
contract.

As future-proofing, define the SQL API to not assume that properties that
are currently AM-wide or index-wide will remain so unless they logically
must be; instead, expose them only when inquiring about a specific index
or even specific index column.  Also provide the ability for an index
AM to override the behavior.

In passing, document pg_am.amtype, overlooked in commit 473b93287.

Andrew Gierth, with kibitzing by me and others

Discussion: <87mvl5on7n.fsf@news-spur.riddles.org.uk>

src/include/access/amapi.h

@@ -26,6 +26,34 @@ struct IndexPath;
 struct IndexInfo;
 
 
+/*
+ * Properties for amproperty API.  This list covers properties known to the
+ * core code, but an index AM can define its own properties, by matching the
+ * string property name.
+ */
+typedef enum IndexAMProperty
+{
+	AMPROP_UNKNOWN = 0,			/* anything not known to core code */
+	AMPROP_ASC,					/* column properties */
+	AMPROP_DESC,
+	AMPROP_NULLS_FIRST,
+	AMPROP_NULLS_LAST,
+	AMPROP_ORDERABLE,
+	AMPROP_DISTANCE_ORDERABLE,
+	AMPROP_RETURNABLE,
+	AMPROP_SEARCH_ARRAY,
+	AMPROP_SEARCH_NULLS,
+	AMPROP_CLUSTERABLE,			/* index properties */
+	AMPROP_INDEX_SCAN,
+	AMPROP_BITMAP_SCAN,
+	AMPROP_BACKWARD_SCAN,
+	AMPROP_CAN_ORDER,			/* AM properties */
+	AMPROP_CAN_UNIQUE,
+	AMPROP_CAN_MULTI_COL,
+	AMPROP_CAN_EXCLUDE
+} IndexAMProperty;
+
+
 /*
  * Callback function signatures --- see indexam.sgml for more info.
  */
@@ -72,6 +100,11 @@ typedef void (*amcostestimate_function) (struct PlannerInfo *root,
 typedef bytea *(*amoptions_function) (Datum reloptions,
 												  bool validate);
 
+/* report AM, index, or index column property */
+typedef bool (*amproperty_function) (Oid index_oid, int attno,
+								  IndexAMProperty prop, const char *propname,
+												 bool *res, bool *isnull);
+
 /* validate definition of an opclass for this AM */
 typedef bool (*amvalidate_function) (Oid opclassoid);
 
@@ -154,6 +187,7 @@ typedef struct IndexAmRoutine
 	amcanreturn_function amcanreturn;	/* can be NULL */
 	amcostestimate_function amcostestimate;
 	amoptions_function amoptions;
+	amproperty_function amproperty;		/* can be NULL */
 	amvalidate_function amvalidate;
 	ambeginscan_function ambeginscan;
 	amrescan_function amrescan;
@@ -167,7 +201,7 @@ typedef struct IndexAmRoutine
 
 /* Functions in access/index/amapi.c */
 extern IndexAmRoutine *GetIndexAmRoutine(Oid amhandler);
-extern IndexAmRoutine *GetIndexAmRoutineByAmId(Oid amoid);
+extern IndexAmRoutine *GetIndexAmRoutineByAmId(Oid amoid, bool noerror);
 
 extern Datum amvalidate(PG_FUNCTION_ARGS);
 

src/include/access/gist_private.h

@@ -492,6 +492,9 @@ extern bool gistvalidate(Oid opclassoid);
 #define GIST_DEFAULT_FILLFACTOR		90
 
 extern bytea *gistoptions(Datum reloptions, bool validate);
+extern bool gistproperty(Oid index_oid, int attno,
+			 IndexAMProperty prop, const char *propname,
+			 bool *res, bool *isnull);
 extern bool gistfitpage(IndexTuple *itvec, int len);
 extern bool gistnospace(Page page, IndexTuple *itvec, int len, OffsetNumber todelete, Size freespace);
 extern void gistcheckpage(Relation rel, Buffer buf);

src/include/access/nbtree.h

@@ -748,6 +748,9 @@ extern void _bt_end_vacuum_callback(int code, Datum arg);
 extern Size BTreeShmemSize(void);
 extern void BTreeShmemInit(void);
 extern bytea *btoptions(Datum reloptions, bool validate);
+extern bool btproperty(Oid index_oid, int attno,
+		   IndexAMProperty prop, const char *propname,
+		   bool *res, bool *isnull);
 
 /*
  * prototypes for functions in nbtvalidate.c