Cache fmgr lookup data for index's getnext() function in IndexScanDesc,

so that the fmgr lookup only has to happen once per index scan and not
once per tuple.  Seems to save 5% or so of CPU time for an indexscan.

src/include/access/relscan.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2000, PostgreSQL, Inc
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: relscan.h,v 1.18 2000/01/26 05:57:51 momjian Exp $
+ * $Id: relscan.h,v 1.19 2000/03/14 23:52:00 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -54,6 +54,7 @@ typedef struct IndexScanDescData
 	bool		scanFromEnd;	/* restart scan at end? */
 	uint16		numberOfKeys;	/* number of key attributes */
 	ScanKey		keyData;		/* key descriptor */
+	FmgrInfo	fn_getnext;		/* cached lookup info for am's getnext fn */
 } IndexScanDescData;
 
 typedef IndexScanDescData *IndexScanDesc;