Support subscripting of arbitrary types, not only arrays.

This patch generalizes the subscripting infrastructure so that any
data type can be subscripted, if it provides a handler function to
define what that means.  Traditional variable-length (varlena) arrays
all use array_subscript_handler(), while the existing fixed-length
types that support subscripting use raw_array_subscript_handler().
It's expected that other types that want to use subscripting notation
will define their own handlers.  (This patch provides no such new
features, though; it only lays the foundation for them.)

To do this, move the parser's semantic processing of subscripts
(including coercion to whatever data type is required) into a
method callback supplied by the handler.  On the execution side,
replace the ExecEvalSubscriptingRef* layer of functions with direct
calls to callback-supplied execution routines.  (Thus, essentially
no new run-time overhead should be caused by this patch.  Indeed,
there is room to remove some overhead by supplying specialized
execution routines.  This patch does a little bit in that line,
but more could be done.)

Additional work is required here and there to remove formerly
hard-wired assumptions about the result type, collation, etc
of a SubscriptingRef expression node; and to remove assumptions
that the subscript values must be integers.

One useful side-effect of this is that we now have a less squishy
mechanism for identifying whether a data type is a "true" array:
instead of wiring in weird rules about typlen, we can look to see
if pg_type.typsubscript == F_ARRAY_SUBSCRIPT_HANDLER.  For this
to be bulletproof, we have to forbid user-defined types from using
that handler directly; but there seems no good reason for them to
do so.

This patch also removes assumptions that the number of subscripts
is limited to MAXDIM (6), or indeed has any hard-wired limit.
That limit still applies to types handled by array_subscript_handler
or raw_array_subscript_handler, but to discourage other dependencies
on this constant, I've moved it from c.h to utils/array.h.

Dmitry Dolgov, reviewed at various times by Tom Lane, Arthur Zakirov,
Peter Eisentraut, Pavel Stehule

Discussion: https://postgr.es/m/CA+q6zcVDuGBv=M0FqBYX8DPebS3F_0KQ6OVFobGJPM507_SZ_w@mail.gmail.com
Discussion: https://postgr.es/m/CA+q6zcVovR+XY4mfk-7oNk-rF91gH0PebnNfuUjuuDsyHjOcVA@mail.gmail.com

1 files changed, 63 insertions, 4 deletions

diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index 47a83658492..7dd49454462 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -2661,8 +2661,9 @@ get_typ_typrelid(Oid typid)
  *
  *		Given the type OID, get the typelem (InvalidOid if not an array type).
  *
- * NB: this only considers varlena arrays to be true arrays; InvalidOid is
- * returned if the input is a fixed-length array type.
+ * NB: this only succeeds for "true" arrays having array_subscript_handler
+ * as typsubscript.  For other types, InvalidOid is returned independently
+ * of whether they have typelem or typsubscript set.
  */
 Oid
 get_element_type(Oid typid)
@@ -2675,7 +2676,7 @@ get_element_type(Oid typid)
 		Form_pg_type typtup = (Form_pg_type) GETSTRUCT(tp);
 		Oid			result;
 
-		if (typtup->typlen == -1)
+		if (IsTrueArrayType(typtup))
 			result = typtup->typelem;
 		else
 			result = InvalidOid;
@@ -2758,7 +2759,7 @@ get_base_element_type(Oid typid)
 			Oid			result;
 
 			/* This test must match get_element_type */
-			if (typTup->typlen == -1)
+			if (IsTrueArrayType(typTup))
 				result = typTup->typelem;
 			else
 				result = InvalidOid;
@@ -2993,6 +2994,64 @@ type_is_collatable(Oid typid)
 }
 
 
+/*
+ * get_typsubscript
+ *
+ *		Given the type OID, return the type's subscripting handler's OID,
+ *		if it has one.
+ *
+ * If typelemp isn't NULL, we also store the type's typelem value there.
+ * This saves some callers an extra catalog lookup.
+ */
+RegProcedure
+get_typsubscript(Oid typid, Oid *typelemp)
+{
+	HeapTuple	tp;
+
+	tp = SearchSysCache1(TYPEOID, ObjectIdGetDatum(typid));
+	if (HeapTupleIsValid(tp))
+	{
+		Form_pg_type typform = (Form_pg_type) GETSTRUCT(tp);
+		RegProcedure handler = typform->typsubscript;
+
+		if (typelemp)
+			*typelemp = typform->typelem;
+		ReleaseSysCache(tp);
+		return handler;
+	}
+	else
+	{
+		if (typelemp)
+			*typelemp = InvalidOid;
+		return InvalidOid;
+	}
+}
+
+/*
+ * getSubscriptingRoutines
+ *
+ *		Given the type OID, fetch the type's subscripting methods struct.
+ *		Fail if type is not subscriptable.
+ *
+ * If typelemp isn't NULL, we also store the type's typelem value there.
+ * This saves some callers an extra catalog lookup.
+ */
+const struct SubscriptRoutines *
+getSubscriptingRoutines(Oid typid, Oid *typelemp)
+{
+	RegProcedure typsubscript = get_typsubscript(typid, typelemp);
+
+	if (!OidIsValid(typsubscript))
+		ereport(ERROR,
+				(errcode(ERRCODE_DATATYPE_MISMATCH),
+				 errmsg("cannot subscript type %s because it does not support subscripting",
+						format_type_be(typid))));
+
+	return (const struct SubscriptRoutines *)
+		DatumGetPointer(OidFunctionCall0(typsubscript));
+}
+
+
 /*				---------- STATISTICS CACHE ----------					 */
 
 /*