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, 30 insertions, 18 deletions

diff --git a/src/backend/parser/parse_target.c b/src/backend/parser/parse_target.c
index ce68663cc2c..3dda8e2847d 100644
--- a/src/backend/parser/parse_target.c
+++ b/src/backend/parser/parse_target.c
@@ -861,7 +861,7 @@ transformAssignmentIndirection(ParseState *pstate,
 		if (targetIsSubscripting)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATATYPE_MISMATCH),
-					 errmsg("array assignment to \"%s\" requires type %s"
+					 errmsg("subscripted assignment to \"%s\" requires type %s"
 							" but expression is of type %s",
 							targetName,
 							format_type_be(targetTypeId),
@@ -901,26 +901,37 @@ transformAssignmentSubscripts(ParseState *pstate,
 							  int location)
 {
 	Node	   *result;
+	SubscriptingRef *sbsref;
 	Oid			containerType;
 	int32		containerTypMod;
-	Oid			elementTypeId;
 	Oid			typeNeeded;
+	int32		typmodNeeded;
 	Oid			collationNeeded;
 
 	Assert(subscripts != NIL);
 
-	/* Identify the actual array type and element type involved */
+	/* Identify the actual container type involved */
 	containerType = targetTypeId;
 	containerTypMod = targetTypMod;
-	elementTypeId = transformContainerType(&containerType, &containerTypMod);
+	transformContainerType(&containerType, &containerTypMod);
 
-	/* Identify type that RHS must provide */
-	typeNeeded = isSlice ? containerType : elementTypeId;
+	/* Process subscripts and identify required type for RHS */
+	sbsref = transformContainerSubscripts(pstate,
+										  basenode,
+										  containerType,
+										  containerTypMod,
+										  subscripts,
+										  true);
+
+	typeNeeded = sbsref->refrestype;
+	typmodNeeded = sbsref->reftypmod;
 
 	/*
-	 * container normally has same collation as elements, but there's an
-	 * exception: we might be subscripting a domain over a container type. In
-	 * that case use collation of the base type.
+	 * Container normally has same collation as its elements, but there's an
+	 * exception: we might be subscripting a domain over a container type.  In
+	 * that case use collation of the base type.  (This is shaky for arbitrary
+	 * subscripting semantics, but it doesn't matter all that much since we
+	 * only use this to label the collation of a possible CaseTestExpr.)
 	 */
 	if (containerType == targetTypeId)
 		collationNeeded = targetCollation;
@@ -933,21 +944,22 @@ transformAssignmentSubscripts(ParseState *pstate,
 										 targetName,
 										 true,
 										 typeNeeded,
-										 containerTypMod,
+										 typmodNeeded,
 										 collationNeeded,
 										 indirection,
 										 next_indirection,
 										 rhs,
 										 location);
 
-	/* process subscripts */
-	result = (Node *) transformContainerSubscripts(pstate,
-												   basenode,
-												   containerType,
-												   elementTypeId,
-												   containerTypMod,
-												   subscripts,
-												   rhs);
+	/*
+	 * Insert the already-properly-coerced RHS into the SubscriptingRef.  Then
+	 * set refrestype and reftypmod back to the container type's values.
+	 */
+	sbsref->refassgnexpr = (Expr *) rhs;
+	sbsref->refrestype = containerType;
+	sbsref->reftypmod = containerTypMod;
+
+	result = (Node *) sbsref;
 
 	/* If target was a domain over container, need to coerce up to the domain */
 	if (containerType != targetTypeId)