Modify btree to delete known-dead index entries without an actual VACUUM.

When we are about to split an index page to do an insertion, first look
to see if any entries marked LP_DELETE exist on the page, and if so remove
them to try to make enough space for the desired insert.  This should reduce
index bloat in heavily-updated tables, although of course you still need
VACUUM eventually to clean up the heap.

Junji Teramoto

src/backend/access/nbtree/nbtpage.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtpage.c,v 1.98 2006/07/13 16:49:12 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtpage.c,v 1.99 2006/07/25 19:13:00 tgl Exp $
  *
  *	NOTES
  *	   Postgres btree pages look like ordinary relation pages.	The opaque
@@ -668,6 +668,15 @@ _bt_delitems(Relation rel, Buffer buf,
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	opaque->btpo_cycleid = 0;
 
+	/*
+	 * Mark the page as not containing any LP_DELETE items.  This is not
+	 * certainly true (there might be some that have recently been marked,
+	 * but weren't included in our target-item list), but it will almost
+	 * always be true and it doesn't seem worth an additional page scan
+	 * to check it.  Remember that BTP_HAS_GARBAGE is only a hint anyway.
+	 */
+	opaque->btpo_flags &= ~BTP_HAS_GARBAGE;
+
 	MarkBufferDirty(buf);
 
 	/* XLOG stuff */