Implement "fastupdate" support for GIN indexes, in which we try to accumulate

multiple index entries in a holding area before adding them to the main index
structure.  This helps because bulk insert is (usually) significantly faster
than retail insert for GIN.

This patch also removes GIN support for amgettuple-style index scans.  The
API defined for amgettuple is difficult to support with fastupdate, and
the previously committed partial-match feature didn't really work with
it either.  We might eventually figure a way to put back amgettuple
support, but it won't happen for 8.4.

catversion bumped because of change in GIN's pg_am entry, and because
the format of GIN indexes changed on-disk (there's a metapage now,
and possibly a pending list).

Teodor Sigaev

src/backend/access/index/indexam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/index/indexam.c,v 1.112 2009/01/01 17:23:35 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/index/indexam.c,v 1.113 2009/03/24 20:17:12 tgl Exp $
  *
  * INTERFACE ROUTINES
  *		index_open		- open an index relation by relation OID
@@ -647,7 +647,8 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
  * item slot could have been replaced by a newer tuple by the time we get
  * to it.
  *
- * Returns the number of matching tuples found.
+ * Returns the number of matching tuples found.  (Note: this might be only
+ * approximate, so it should only be used for statistical purposes.)
  * ----------------
  */
 int64