A couple of tiny performance hacks in _bt_step(). Remove PageIsEmpty

checks, which were once needed because PageGetMaxOffsetNumber would
fail on empty pages, but are now just redundant.  Also, don't set up
local variables that aren't needed in the fast path --- most of the
time, we only need to advance offnum and not step across a page boundary.
Motivated by noticing _bt_step at the top of OProfile profile for a
pgbench run.

Author: tglsfdc

src/backend/access/nbtree/nbtsearch.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.97 2005/11/22 18:17:06 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.98 2005/12/07 18:03:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -889,9 +889,9 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 bool
 _bt_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 {
-	Relation	rel = scan->indexRelation;
 	ItemPointer current = &(scan->currentItemData);
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
+	Relation	rel;
 	Page		page;
 	BTPageOpaque opaque;
 	OffsetNumber offnum,
@@ -905,16 +905,17 @@ _bt_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 	offnum = current->ip_posid;
 
 	page = BufferGetPage(*bufP);
-	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	maxoff = PageGetMaxOffsetNumber(page);
 
 	if (ScanDirectionIsForward(dir))
 	{
-		if (!PageIsEmpty(page) && offnum < maxoff)
+		if (offnum < maxoff)
 			offnum = OffsetNumberNext(offnum);
 		else
 		{
 			/* Walk right to the next page with data */
+			rel = scan->indexRelation;
+			opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 			for (;;)
 			{
 				/* if we're at end of scan, release the buffer and return */
@@ -932,10 +933,10 @@ _bt_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 				opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 				if (!P_IGNORE(opaque))
 				{
-					maxoff = PageGetMaxOffsetNumber(page);
 					/* done if it's not empty */
+					maxoff = PageGetMaxOffsetNumber(page);
 					offnum = P_FIRSTDATAKEY(opaque);
-					if (!PageIsEmpty(page) && offnum <= maxoff)
+					if (offnum <= maxoff)
 						break;
 				}
 			}
@@ -944,6 +945,7 @@ _bt_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 	else
 	{
 		/* backwards scan */
+		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		if (offnum > P_FIRSTDATAKEY(opaque))
 			offnum = OffsetNumberPrev(offnum);
 		else
@@ -955,6 +957,7 @@ _bt_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 			 * plus the possibility that the page we were on gets deleted
 			 * after we leave it.  See nbtree/README for details.
 			 */
+			rel = scan->indexRelation;
 			for (;;)
 			{
 				*bufP = _bt_walk_left(rel, *bufP);
@@ -978,8 +981,7 @@ _bt_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 				{
 					maxoff = PageGetMaxOffsetNumber(page);
 					offnum = maxoff;
-					if (!PageIsEmpty(page) &&
-						maxoff >= P_FIRSTDATAKEY(opaque))
+					if (maxoff >= P_FIRSTDATAKEY(opaque))
 						break;
 				}
 			}