--- pgsql.orig/src/backend/catalog/heap.c	2007-05-15 09:34:25.000000000 +0200
+++ pgsql/src/backend/catalog/heap.c	2007-05-18 21:33:04.000000000 +0200
@@ -1935,6 +1935,43 @@
 			  errmsg("cannot use column references in default expression")));
 
 	/*
+	 * Make sure default expr may contain only
+	 * standard compliant functions as of SQL:2003:
+	 * - CURRENT_DATE
+	 * - CURRENT_TIME[ ( precision ) ]
+	 * - CURRENT_TIMESTAMP[ ( precision ) ]
+	 * - LOCALTIME[ ( precision ) ]
+	 * - LOCALTIMESTAMP[ ( precision ) ]
+	 * - as a PostgreSQL extension,
+	 *   all others that call now() implicitely or explicitely
+	 * - USER
+	 * - CURRENT_USER
+	 * - CURRENT_ROLE
+	 * - SESSION_USER
+	 * with two other PostgreSQL extensions:
+	 * - nextval() so SERIALs work
+	 * - any immutable functions to pave the way for GENERATED columns
+	 * Please note that PostgreSQL lacks SYSTEM_USER and CURRENT_PATH.
+	 */
+	if (is_opclause(expr)) {
+		OpExpr *clause = (OpExpr *)expr;
+
+		switch (clause->opfuncid)
+		{
+			case 745:	/* current_user */
+			case 746:	/* session_user */
+			case 1299:	/* now() */
+			case 1574:	/* nextval() */
+				break;
+			default:
+				if (contain_mutable_functions(expr))
+					ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						  errmsg("cannot use non-IMMUTABLE functions in default expression")));
+		}
+	}
+
+	/*
 	 * It can't return a set either.
 	 */
 	if (expression_returns_set(expr))