Make eval_const_expressions() preserve typmod when simplifying something like

null::char(3) to a simple Const node.  (It already worked for non-null values,
but not when we skipped evaluation of a strict coercion function.)  This
prevents loss of typmod knowledge in situations such as exhibited in bug
#3598.  Unfortunately there seems no good way to fix that bug in 8.1 and 8.2,
because they simply don't carry a typmod for a plain Const node.

In passing I made all the other callers of makeNullConst supply "real" typmod
values too, though I think it probably doesn't matter anywhere else.

src/backend/rewrite/rewriteManip.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/rewrite/rewriteManip.c,v 1.104 2007/06/11 01:16:25 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/rewrite/rewriteManip.c,v 1.105 2007/09/06 17:31:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -896,7 +896,8 @@ resolve_one_var(Var *var, ResolveNew_context *context)
 		{
 			/* Otherwise replace unmatched var with a null */
 			/* need coerce_to_domain in case of NOT NULL domain constraint */
-			return coerce_to_domain((Node *) makeNullConst(var->vartype),
+			return coerce_to_domain((Node *) makeNullConst(var->vartype,
+														   var->vartypmod),
 									InvalidOid, -1,
 									var->vartype,
 									COERCE_IMPLICIT_CAST,