From f1b059af12673abf93ba13594799732bac83ed01 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 7 Dec 2005 18:03:48 +0000
Subject: [PATCH] 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.

---
 src/backend/access/nbtree/nbtsearch.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 9c42797b95..dd4f500842 100644
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/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;
 				}
 			}
-- 
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