Have CLUSTER advance the table's relfrozenxid. The new frozen point is the

FreezeXid introduced in a recent commit, so there isn't any data loss in this
approach.

Doing it causes ALTER TABLE (or rather, the forms of it that cause a full table
rewrite) to be affected as well.  In this case, the frozen point is RecentXmin,
because after the rewrite all the tuples are relabeled with the rewriting
transaction's Xid.

TOAST tables are fixed automatically as well, as fallout of the way they were
already being handled in the respective code paths.

With this patch, there is no longer need to VACUUM tables for Xid wraparound
purposes that have been cleaned up via TRUNCATE or CLUSTER.

src/backend/commands/cluster.c

@@ -11,7 +11,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/cluster.c,v 1.160 2007/05/17 15:28:29 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/cluster.c,v 1.161 2007/05/18 23:19:41 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -55,7 +55,7 @@ typedef struct
 
 static void cluster_rel(RelToCluster *rv, bool recheck);
 static void rebuild_relation(Relation OldHeap, Oid indexOid);
-static void copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex);
+static TransactionId copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex);
 static List *get_tables_to_cluster(MemoryContext cluster_context);
 
 
@@ -513,6 +513,7 @@ rebuild_relation(Relation OldHeap, Oid indexOid)
 	Oid			tableSpace = OldHeap->rd_rel->reltablespace;
 	Oid			OIDNewHeap;
 	char		NewHeapName[NAMEDATALEN];
+	TransactionId frozenXid;
 	ObjectAddress object;
 
 	/* Mark the correct index as clustered */
@@ -539,13 +540,13 @@ rebuild_relation(Relation OldHeap, Oid indexOid)
 	/*
 	 * Copy the heap data into the new table in the desired order.
 	 */
-	copy_heap_data(OIDNewHeap, tableOid, indexOid);
+	frozenXid = copy_heap_data(OIDNewHeap, tableOid, indexOid);
 
 	/* To make the new heap's data visible (probably not needed?). */
 	CommandCounterIncrement();
 
 	/* Swap the physical files of the old and new heaps. */
-	swap_relation_files(tableOid, OIDNewHeap);
+	swap_relation_files(tableOid, OIDNewHeap, frozenXid);
 
 	CommandCounterIncrement();
 
@@ -641,9 +642,10 @@ make_new_heap(Oid OIDOldHeap, const char *NewName, Oid NewTableSpace)
 }
 
 /*
- * Do the physical copying of heap data.
+ * Do the physical copying of heap data.  Returns the TransactionId used as
+ * freeze cutoff point for the tuples.
  */
-static void
+static TransactionId
 copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex)
 {
 	Relation	NewHeap,
@@ -816,6 +818,8 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex)
 	index_close(OldIndex, NoLock);
 	heap_close(OldHeap, NoLock);
 	heap_close(NewHeap, NoLock);
+
+	return FreezeXid;
 }
 
 /*
@@ -826,9 +830,16 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex)
  *
  * Also swap any TOAST links, so that the toast data moves along with
  * the main-table data.
+ *
+ * Additionally, the first relation is marked with relfrozenxid set to
+ * frozenXid.  It seems a bit ugly to have this here, but all callers would
+ * have to do it anyway, so having it here saves a heap_update.  Note: the
+ * TOAST table needs no special handling, because since we swapped the links,
+ * the entry for the TOAST table will now contain RecentXmin in relfrozenxid,
+ * which is the correct value.
  */
 void
-swap_relation_files(Oid r1, Oid r2)
+swap_relation_files(Oid r1, Oid r2, TransactionId frozenXid)
 {
 	Relation	relRelation;
 	HeapTuple	reltup1,
@@ -872,6 +883,10 @@ swap_relation_files(Oid r1, Oid r2)
 
 	/* we should not swap reltoastidxid */
 
+	/* set rel1's frozen Xid */
+	Assert(TransactionIdIsNormal(frozenXid));
+	relform1->relfrozenxid = frozenXid;
+
 	/* swap size statistics too, since new rel has freshly-updated stats */
 	{
 		int4		swap_pages;

src/backend/commands/tablecmds.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.224 2007/05/16 17:28:20 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.225 2007/05/18 23:19:41 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2285,8 +2285,13 @@ ATRewriteTables(List **wqueue)
 			 */
 			ATRewriteTable(tab, OIDNewHeap);
 
-			/* Swap the physical files of the old and new heaps. */
-			swap_relation_files(tab->relid, OIDNewHeap);
+			/*
+			 * Swap the physical files of the old and new heaps.  Since we are
+			 * generating a new heap, we can use RecentXmin for the table's new
+			 * relfrozenxid because we rewrote all the tuples on
+			 * ATRewriteTable, so no older Xid remains on the table.
+			 */
+			swap_relation_files(tab->relid, OIDNewHeap, RecentXmin);
 
 			CommandCounterIncrement();
 

src/include/commands/cluster.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994-5, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/commands/cluster.h,v 1.32 2007/03/13 00:33:43 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/commands/cluster.h,v 1.33 2007/05/18 23:19:42 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -24,6 +24,6 @@ extern void check_index_is_clusterable(Relation OldHeap, Oid indexOid,
 extern void mark_index_clustered(Relation rel, Oid indexOid);
 extern Oid make_new_heap(Oid OIDOldHeap, const char *NewName,
 			  Oid NewTableSpace);
-extern void swap_relation_files(Oid r1, Oid r2);
+extern void swap_relation_files(Oid r1, Oid r2, TransactionId frozenXid);
 
 #endif   /* CLUSTER_H */