Skip ambulkdelete scan if there's nothing to delete and the index is not

partial.  None of the existing AMs do anything useful except counting
tuples when there's nothing to delete, and we can get a tuple count
from the heap as long as it's not a partial index.  (hash actually can
skip anyway because it maintains a tuple count in the index metapage.)
GIST is not currently able to exploit this optimization because, due to
failure to index NULLs, GIST is always effectively partial.  Possibly
we should fix that sometime.
Simon Riggs w/ some review by Tom Lane.

src/include/access/genam.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/genam.h,v 1.56 2006/02/11 17:14:09 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/access/genam.h,v 1.57 2006/02/11 23:31:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -51,6 +51,7 @@ typedef struct IndexVacuumCleanupInfo
 {
 	bool		vacuum_full;	/* VACUUM FULL (we have exclusive lock) */
 	int			message_level;	/* ereport level for progress messages */
+	double		num_heap_tuples;	/* tuples remaining in heap */
 } IndexVacuumCleanupInfo;
 
 /* Struct for heap-or-index scans of system tables */