Restructure CLUSTER/newstyle VACUUM FULL/ALTER TABLE support so that swapping

of old and new toast tables can be done either at the logical level (by
swapping the heaps' reltoastrelid links) or at the physical level (by swapping
the relfilenodes of the toast tables and their indexes).  This is necessary
infrastructure for upcoming changes to support CLUSTER/VAC FULL on shared
system catalogs, where we cannot change reltoastrelid.  The physical swap
saves a few catalog updates too.

We unfortunately have to keep the logical-level swap logic because in some
cases we will be adding or deleting a toast table, so there's no possibility
of a physical swap.  However, that only happens as a consequence of schema
changes in the table, which we do not need to support for system catalogs,
so such cases aren't an obstacle for that.

In passing, refactor the cluster support functions a little bit to eliminate
unnecessarily-duplicated code; and fix the problem that while CLUSTER had
been taught to rename the final toast table at need, ALTER TABLE had not.

Author: tglsfdc

src/backend/access/heap/tuptoaster.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/heap/tuptoaster.c,v 1.96 2010/01/02 16:57:35 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/heap/tuptoaster.c,v 1.97 2010/02/04 00:09:13 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -1185,10 +1185,25 @@ toast_save_datum(Relation rel, Datum value, int options)
 		toast_pointer.va_extsize = data_todo;
 	}
 
+	/*
+	 * Insert the correct table OID into the result TOAST pointer.
+	 *
+	 * Normally this is the actual OID of the target toast table, but during
+	 * table-rewriting operations such as CLUSTER, we have to insert the OID
+	 * of the table's real permanent toast table instead.  rd_toastoid is
+	 * set if we have to substitute such an OID.
+	 */
+	if (OidIsValid(rel->rd_toastoid))
+		toast_pointer.va_toastrelid = rel->rd_toastoid;
+	else
+		toast_pointer.va_toastrelid = RelationGetRelid(toastrel);
+
+	/*
+	 * Choose an unused OID within the toast table for this toast value.
+	 */
 	toast_pointer.va_valueid = GetNewOidWithIndex(toastrel,
 												  RelationGetRelid(toastidx),
 												  (AttrNumber) 1);
-	toast_pointer.va_toastrelid = rel->rd_rel->reltoastrelid;
 
 	/*
 	 * Initialize constant parts of the tuple data