Optimize the case where a btree indexscan has current and mark positions

on the same index page; we can avoid data copying as well as buffer refcount
manipulations in this common case.  Makes for a small but noticeable
improvement in mergejoin speed.

Heikki Linnakangas

Author: tglsfdc

src/backend/access/nbtree/nbtree.c

@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtree.c,v 1.149 2006/05/10 23:18:39 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtree.c,v 1.150 2006/08/24 01:18:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -392,6 +392,7 @@ btrescan(PG_FUNCTION_ARGS)
 		ReleaseBuffer(so->markPos.buf);
 		so->markPos.buf = InvalidBuffer;
 	}
+	so->markItemIndex = -1;
 
 	/*
 	 * Reset the scan keys. Note that keys ordering stuff moved to _bt_first.
@@ -430,6 +431,7 @@ btendscan(PG_FUNCTION_ARGS)
 		ReleaseBuffer(so->markPos.buf);
 		so->markPos.buf = InvalidBuffer;
 	}
+	so->markItemIndex = -1;
 
 	if (so->killedItems != NULL)
 		pfree(so->killedItems);
@@ -456,14 +458,16 @@ btmarkpos(PG_FUNCTION_ARGS)
 		so->markPos.buf = InvalidBuffer;
 	}
 
-	/* bump pin on current buffer for assignment to mark buffer */
+	/*
+	 * Just record the current itemIndex.  If we later step to next page
+	 * before releasing the marked position, _bt_steppage makes a full copy
+	 * of the currPos struct in markPos.  If (as often happens) the mark is
+	 * moved before we leave the page, we don't have to do that work.
+	 */
 	if (BTScanPosIsValid(so->currPos))
-	{
-		IncrBufferRefCount(so->currPos.buf);
-		memcpy(&so->markPos, &so->currPos,
-			   offsetof(BTScanPosData, items[1]) +
-			   so->currPos.lastItem * sizeof(BTScanPosItem));
-	}
+		so->markItemIndex = so->currPos.itemIndex;
+	else
+		so->markItemIndex = -1;
 
 	PG_RETURN_VOID();
 }
@@ -477,24 +481,35 @@ btrestrpos(PG_FUNCTION_ARGS)
 	IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
-	/* we aren't holding any read locks, but gotta drop the pin */
-	if (BTScanPosIsValid(so->currPos))
+	if (so->markItemIndex >= 0)
 	{
-		/* Before leaving current page, deal with any killed items */
-		if (so->numKilled > 0 &&
-			so->currPos.buf != so->markPos.buf)
-			_bt_killitems(scan, false);
-		ReleaseBuffer(so->currPos.buf);
-		so->currPos.buf = InvalidBuffer;
-	}
-
-	/* bump pin on marked buffer */
-	if (BTScanPosIsValid(so->markPos))
+		/*
+		 * The mark position is on the same page we are currently on.
+		 * Just restore the itemIndex.
+		 */
+		so->currPos.itemIndex = so->markItemIndex;
+	} 
+	else
 	{
-		IncrBufferRefCount(so->markPos.buf);
-		memcpy(&so->currPos, &so->markPos,
-			   offsetof(BTScanPosData, items[1]) +
-			   so->markPos.lastItem * sizeof(BTScanPosItem));
+		/* we aren't holding any read locks, but gotta drop the pin */
+		if (BTScanPosIsValid(so->currPos))
+		{
+			/* Before leaving current page, deal with any killed items */
+			if (so->numKilled > 0 &&
+				so->currPos.buf != so->markPos.buf)
+				_bt_killitems(scan, false);
+			ReleaseBuffer(so->currPos.buf);
+			so->currPos.buf = InvalidBuffer;
+		}
+
+		if (BTScanPosIsValid(so->markPos))
+		{
+			/* bump pin on mark buffer for assignment to current buffer */
+			IncrBufferRefCount(so->markPos.buf);
+			memcpy(&so->currPos, &so->markPos,
+				   offsetof(BTScanPosData, items[1]) +
+				   so->markPos.lastItem * sizeof(BTScanPosItem));
+		}
 	}
 
 	PG_RETURN_VOID();

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 1.105 2006/05/07 01:21:30 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.106 2006/08/24 01:18:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -815,6 +815,7 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 		so->currPos.moreRight = false;
 	}
 	so->numKilled = 0;			/* just paranoia */
+	so->markItemIndex = -1;		/* ditto */
 
 	/* position to the precise item on the page */
 	offnum = _bt_binsrch(rel, buf, keysCount, scankeys, nextkey);
@@ -1053,6 +1054,21 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
 	if (so->numKilled > 0)
 		_bt_killitems(scan, true);
 
+	/*
+	 * Before we modify currPos, make a copy of the page data if there
+	 * was a mark position that needs it.
+	 */
+	if (so->markItemIndex >= 0)
+	{
+		/* bump pin on current buffer for assignment to mark buffer */
+		IncrBufferRefCount(so->currPos.buf);
+		memcpy(&so->markPos, &so->currPos,
+			   offsetof(BTScanPosData, items[1]) +
+			   so->currPos.lastItem * sizeof(BTScanPosItem));
+		so->markPos.itemIndex = so->markItemIndex;
+		so->markItemIndex = -1;
+	}
+
 	rel = scan->indexRelation;
 
 	if (ScanDirectionIsForward(dir))
@@ -1408,6 +1424,7 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
 		so->currPos.moreRight = false;
 	}
 	so->numKilled = 0;			/* just paranoia */
+	so->markItemIndex = -1;		/* ditto */
 
 	/*
 	 * Now load data from the first page of the scan.

src/include/access/nbtree.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/nbtree.h,v 1.103 2006/08/07 16:57:57 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/nbtree.h,v 1.104 2006/08/24 01:18:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -438,6 +438,15 @@ typedef struct BTScanOpaqueData
 	int		   *killedItems;	/* currPos.items indexes of killed items */
 	int			numKilled;		/* number of currently stored items */
 
+	/*
+	 * If the marked position is on the same page as current position,
+	 * we don't use markPos, but just keep the marked itemIndex in
+	 * markItemIndex (all the rest of currPos is valid for the mark position).
+	 * Hence, to determine if there is a mark, first look at markItemIndex,
+	 * then at markPos.
+	 */
+	int			markItemIndex;	/* itemIndex, or -1 if not valid */
+
 	/* keep these last in struct for efficiency */
 	BTScanPosData currPos;		/* current position data */
 	BTScanPosData markPos;		/* marked position, if any */