Clean up handling of explicit NULL constants. Cases like

	SELECT null::text;
	SELECT int4fac(null);
work as expected now.  In some cases a NULL must be surrounded by
parentheses:
	SELECT 2 + null;                 fails
	SELECT 2 + (null);               OK
This is a grammatical ambiguity that seems difficult to avoid.  Other
than that, NULLs seem to behave about like you'd expect.  The internal
implementation is that NULL constants are typed as UNKNOWN (like
untyped string constants) until the parser can deduce the right type.

src/backend/nodes/equalfuncs.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/nodes/equalfuncs.c,v 1.53 1999/12/13 01:26:53 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/nodes/equalfuncs.c,v 1.54 1999/12/24 06:43:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -144,6 +144,13 @@ _equalConst(Const *a, Const *b)
 	if (a->constbyval != b->constbyval)
 		return false;
 	/* XXX What about constisset and constiscast? */
+	/*
+	 * We treat all NULL constants of the same type as equal.
+	 * Someday this might need to change?  But datumIsEqual
+	 * doesn't work on nulls, so...
+	 */
+	if (a->constisnull)
+		return true;
 	return (datumIsEqual(a->constvalue, b->constvalue,
 						 a->consttype, a->constbyval, a->constlen));
 }

src/backend/parser/parse_expr.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/parser/parse_expr.c,v 1.62 1999/12/17 01:25:25 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/parser/parse_expr.c,v 1.63 1999/12/24 06:43:33 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -353,7 +353,7 @@ transformExpr(ParseState *pstate, Node *expr, int precedence)
 					n->val.type = T_Null;
 					c->defresult = (Node *) n;
 				}
-				c->defresult = transformExpr(pstate, (Node *) c->defresult, precedence);
+				c->defresult = transformExpr(pstate, c->defresult, precedence);
 
 				/* now check types across result clauses... */
 				c->casetype = exprType(c->defresult);
@@ -369,32 +369,30 @@ transformExpr(ParseState *pstate, Node *expr, int precedence)
 					if (wtype && (wtype != UNKNOWNOID)
 						&& (wtype != ptype))
 					{
-						/* so far, only nulls so take anything... */
-						if (!ptype)
+						if (!ptype || ptype == UNKNOWNOID)
 						{
+							/* so far, only nulls so take anything... */
 							ptype = wtype;
 							pcategory = TypeCategory(ptype);
 						}
-
-						/*
-						 * both types in different categories? then not
-						 * much hope...
-						 */
 						else if ((TypeCategory(wtype) != pcategory)
 								 || ((TypeCategory(wtype) == USER_TYPE)
 							&& (TypeCategory(c->casetype) == USER_TYPE)))
 						{
+							/*
+							 * both types in different categories?
+							 * then not much hope...
+							 */
 							elog(ERROR, "CASE/WHEN types '%s' and '%s' not matched",
 								 typeidTypeName(c->casetype), typeidTypeName(wtype));
 						}
-
-						/*
-						 * new one is preferred and can convert? then take
-						 * it...
-						 */
 						else if (IsPreferredType(pcategory, wtype)
 								 && can_coerce_type(1, &ptype, &wtype))
 						{
+							/*
+							 * new one is preferred and can convert?
+							 * then take it...
+							 */
 							ptype = wtype;
 							pcategory = TypeCategory(ptype);
 						}
@@ -404,9 +402,8 @@ transformExpr(ParseState *pstate, Node *expr, int precedence)
 				/* Convert default result clause, if necessary */
 				if (c->casetype != ptype)
 				{
-					if (!c->casetype)
+					if (!c->casetype || c->casetype == UNKNOWNOID)
 					{
-
 						/*
 						 * default clause is NULL, so assign preferred
 						 * type from WHEN clauses...
@@ -694,11 +691,12 @@ exprTypmod(Node *expr)
 static Node *
 parser_typecast(Value *expr, TypeName *typename, int32 atttypmod)
 {
-	Const	   *adt;
-	Datum		lcp;
+	Const	   *con;
 	Type		tp;
+	Datum		datum;
 	char	   *const_string = NULL;
 	bool		string_palloced = false;
+	bool		isNull = false;
 
 	switch (nodeTag(expr))
 	{
@@ -713,6 +711,9 @@ parser_typecast(Value *expr, TypeName *typename, int32 atttypmod)
 			string_palloced = true;
 			const_string = float8out(&expr->val.dval);
 			break;
+		case T_Null:
+			isNull = true;
+			break;
 		default:
 			elog(ERROR,
 				 "parser_typecast: cannot cast this expression to type '%s'",
@@ -729,12 +730,15 @@ parser_typecast(Value *expr, TypeName *typename, int32 atttypmod)
 	else
 		tp = (Type) typenameType(typename->name);
 
-	lcp = stringTypeDatum(tp, const_string, atttypmod);
+	if (isNull)
+		datum = (Datum) NULL;
+	else
+		datum = stringTypeDatum(tp, const_string, atttypmod);
 
-	adt = makeConst(typeTypeId(tp),
+	con = makeConst(typeTypeId(tp),
 					typeLen(tp),
-					(Datum) lcp,
-					false,
+					datum,
+					isNull,
 					typeByVal(tp),
 					false,		/* not a set */
 					true /* is cast */ );
@@ -742,5 +746,5 @@ parser_typecast(Value *expr, TypeName *typename, int32 atttypmod)
 	if (string_palloced)
 		pfree(const_string);
 
-	return (Node *) adt;
+	return (Node *) con;
 }

src/backend/parser/parse_node.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/parser/parse_node.c,v 1.33 1999/11/22 17:56:21 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/parser/parse_node.c,v 1.34 1999/12/24 06:43:33 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -61,28 +61,27 @@ make_operand(char *opname,
 			 Oid target_typeId)
 {
 	Node	   *result;
-	Type		target_type;
+	Type		target_type = typeidType(target_typeId);
 
 	if (tree != NULL)
 	{
-		result = tree;
-		target_type = typeidType(target_typeId);
-		disallow_setop(opname, target_type, result);
-
+		disallow_setop(opname, target_type, tree);
 		/* must coerce? */
 		if (target_typeId != orig_typeId)
 			result = coerce_type(NULL, tree, orig_typeId, target_typeId, -1);
+		else
+			result = tree;
 	}
-	/* otherwise, this is a NULL value */
 	else
 	{
+		/* otherwise, this is a NULL value */
 		Const	   *con = makeNode(Const);
 
 		con->consttype = target_typeId;
-		con->constlen = 0;
-		con->constvalue = (Datum) (struct varlena *) NULL;
+		con->constlen = typeLen(target_type);
+		con->constvalue = (Datum) NULL;
 		con->constisnull = true;
-		con->constbyval = true;
+		con->constbyval = typeByVal(target_type);
 		con->constisset = false;
 		result = (Node *) con;
 	}
@@ -394,7 +393,8 @@ transformArraySubscripts(ParseState *pstate,
  *	of the "natural" type for the constant.  For strings we produce
  *	a constant of type UNKNOWN ---- representation is the same as text,
  *	but this indicates to later type resolution that we're not sure that
- *	it should be considered text.
+ *	it should be considered text.  Explicit "NULL" constants are also
+ *	typed as UNKNOWN.
  */
 Const *
 make_const(Value *value)
@@ -444,7 +444,13 @@ make_const(Value *value)
 				elog(NOTICE, "make_const: unknown type %d\n", nodeTag(value));
 
 			/* return a null const */
-			con = makeConst(0, 0, (Datum) NULL, true, false, false, false);
+			con = makeConst(UNKNOWNOID,
+							-1,
+							(Datum) NULL,
+							true,
+							false,
+							false,
+							false);
 			return con;
 	}
 

src/backend/utils/adt/ruleutils.c

@@ -3,7 +3,7 @@
  *			  out of it's tuple
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/utils/adt/ruleutils.c,v 1.35 1999/12/13 01:27:01 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/utils/adt/ruleutils.c,v 1.36 1999/12/24 06:43:34 tgl Exp $
  *
  *	  This software is copyrighted by Jan Wieck - Hamburg.
  *
@@ -1606,12 +1606,6 @@ get_const_expr(Const *constval, deparse_context *context)
 	char	   *valptr;
 	bool		isnull = FALSE;
 
-	if (constval->constisnull)
-	{
-		appendStringInfo(buf, "NULL");
-		return;
-	}
-
 	typetup = SearchSysCacheTuple(TYPEOID,
 								  ObjectIdGetDatum(constval->consttype),
 								  0, 0, 0);
@@ -1620,6 +1614,19 @@ get_const_expr(Const *constval, deparse_context *context)
 
 	typeStruct = (Form_pg_type) GETSTRUCT(typetup);
 
+	if (constval->constisnull)
+	{
+		/*
+		 * Always label the type of a NULL constant.  This not only
+		 * prevents misdecisions about the type, but it ensures that
+		 * our output is a valid b_expr.
+		 */
+		extval = pstrdup(NameStr(typeStruct->typname));
+		appendStringInfo(buf, "NULL::%s", quote_identifier(extval));
+		pfree(extval);
+		return;
+	}
+
 	fmgr_info(typeStruct->typoutput, &finfo_output);
 	extval = (char *) (*fmgr_faddr(&finfo_output)) (constval->constvalue,
 													&isnull, -1);