Improve the IndexVacuumInfo/IndexBulkDeleteResult API to allow somewhat sane

behavior in cases where we don't know the heap tuple count accurately; in
particular partial vacuum, but this also makes the API a bit more useful
for ANALYZE.  This patch adds "estimated_count" flags to both structs so
that an approximate count can be flagged as such, and adjusts the logic
so that approximate counts are not used for updating pg_class.reltuples.

This fixes my previous complaint that VACUUM was putting ridiculous values
into pg_class.reltuples for indexes.  The actual impact of that bug is
limited, because the planner only pays attention to reltuples for an index
if the index is partial; which probably explains why beta testers hadn't
noticed a degradation in plan quality from it.  But it needs to be fixed.

The whole thing is a bit messy and should be redesigned in future, because
reltuples now has the potential to drift quite far away from reality when
a long period elapses with no non-partial vacuums.  But this is as good as
it's going to get for 8.4.

src/include/access/genam.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/genam.h,v 1.76 2009/03/24 20:17:14 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/genam.h,v 1.77 2009/06/06 22:13:52 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -34,14 +34,17 @@ typedef struct IndexBuildResult
 /*
  * Struct for input arguments passed to ambulkdelete and amvacuumcleanup
  *
- * Note that num_heap_tuples will not be valid during ambulkdelete,
- * only amvacuumcleanup.
+ * num_heap_tuples is accurate only when estimated_count is false;
+ * otherwise it's just an estimate (currently, the estimate is the
+ * prior value of the relation's pg_class.reltuples field).  It will
+ * always just be an estimate during ambulkdelete.
  */
 typedef struct IndexVacuumInfo
 {
 	Relation	index;			/* the index being vacuumed */
 	bool		vacuum_full;	/* VACUUM FULL (we have exclusive lock) */
 	bool		analyze_only;	/* ANALYZE (without any actual vacuum) */
+	bool		estimated_count;	/* num_heap_tuples is an estimate */
 	int			message_level;	/* ereport level for progress messages */
 	double		num_heap_tuples;	/* tuples remaining in heap */
 	BufferAccessStrategy strategy;		/* access strategy for reads */
@@ -60,12 +63,15 @@ typedef struct IndexVacuumInfo
  *
  * Note: pages_removed is the amount by which the index physically shrank,
  * if any (ie the change in its total size on disk).  pages_deleted and
- * pages_free refer to free space within the index file.
+ * pages_free refer to free space within the index file.  Some index AMs
+ * may compute num_index_tuples by reference to num_heap_tuples, in which
+ * case they should copy the estimated_count field from IndexVacuumInfo.
  */
 typedef struct IndexBulkDeleteResult
 {
 	BlockNumber num_pages;		/* pages remaining in index */
 	BlockNumber pages_removed;	/* # removed during vacuum operation */
+	bool		estimated_count;	/* num_index_tuples is an estimate */
 	double		num_index_tuples;		/* tuples remaining */
 	double		tuples_removed; /* # removed during vacuum operation */
 	BlockNumber pages_deleted;	/* # unused pages in index */