Have the rewriteheap code freeze old tuples. This is safe because it is only

applied to live tuples older than a recent Xmin, not to tuples that may be part
of an update chain.  Those still keep their original markings.

This patch makes it possible for CLUSTER to advance relfrozenxid, thus avoiding
the need of vacuuming the table for Xid wraparound purposes.  That will be
patched separately.

Patch from Heikki Linnakangas.

src/backend/access/heap/rewriteheap.c

@@ -96,7 +96,7 @@
  * Portions Copyright (c) 1994-5, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/heap/rewriteheap.c,v 1.3 2007/04/17 21:29:31 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/heap/rewriteheap.c,v 1.4 2007/05/16 16:36:56 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -538,6 +538,8 @@ raw_heap_insert(RewriteState state, HeapTuple tup)
 	OffsetNumber	newoff;
 	HeapTuple		heaptup;
 
+	heap_freeze_tuple(tup->t_data, state->rs_oldest_xmin, InvalidBuffer);
+
 	/*
 	 * If the new tuple is too big for storage or contains already toasted
 	 * out-of-line attributes from some other relation, invoke the toaster.