Fix recently-understood problems with handling of XID freezing, particularly

in PITR scenarios.  We now WAL-log the replacement of old XIDs with
FrozenTransactionId, so that such replacement is guaranteed to propagate to
PITR slave databases.  Also, rather than relying on hint-bit updates to be
preserved, pg_clog is not truncated until all instances of an XID are known to
have been replaced by FrozenTransactionId.  Add new GUC variables and
pg_autovacuum columns to allow management of the freezing policy, so that
users can trade off the size of pg_clog against the amount of freezing work
done.  Revise the already-existing code that forces autovacuum of tables
approaching the wraparound point to make it more bulletproof; also, revise the
autovacuum logic so that anti-wraparound vacuuming is done per-table rather
than per-database.  initdb forced because of changes in pg_class, pg_database,
and pg_autovacuum catalogs.  Heikki Linnakangas, Simon Riggs, and Tom Lane.

src/include/access/htup.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.86 2006/10/04 00:30:07 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.87 2006/11/05 22:42:10 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -510,6 +510,13 @@ typedef HeapTupleData *HeapTuple;
  * we can (and we do) restore entire page in redo
  */
 #define XLOG_HEAP_INIT_PAGE 0x80
+/*
+ * We ran out of opcodes, so heapam.c now has a second RmgrId.  These opcodes
+ * are associated with RM_HEAP2_ID, but are not logically different from
+ * the ones above associated with RM_HEAP_ID.  We apply XLOG_HEAP_OPMASK,
+ * although currently XLOG_HEAP_INIT_PAGE is not used for any of these.
+ */
+#define XLOG_HEAP2_FREEZE	0x00
 
 /*
  * All what we need to find changed tuple
@@ -613,4 +620,15 @@ typedef struct xl_heap_inplace
 
 #define SizeOfHeapInplace	(offsetof(xl_heap_inplace, target) + SizeOfHeapTid)
 
+/* This is what we need to know about tuple freezing during vacuum */
+typedef struct xl_heap_freeze
+{
+	RelFileNode node;
+	BlockNumber block;
+	TransactionId cutoff_xid;
+	/* TUPLE OFFSET NUMBERS FOLLOW AT THE END */
+} xl_heap_freeze;
+
+#define SizeOfHeapFreeze (offsetof(xl_heap_freeze, cutoff_xid) + sizeof(TransactionId))
+
 #endif   /* HTUP_H */