Improve ExecStoreTuple to be smarter about replacing the contents of

a TupleTableSlot: instead of calling ExecClearTuple, inline the needed
operations, so that we can avoid redundant steps.  In particular, when
the old and new tuples are both on the same disk page, avoid releasing
and re-acquiring the buffer pin --- this saves work in both the bufmgr
and ResourceOwner modules.  To make this improvement actually useful,
partially revert a change I made on 2004-04-21 that caused SeqNext
et al to call ExecClearTuple before ExecStoreTuple.  The motivation
for that, to avoid grabbing the BufMgrLock separately for releasing
the old buffer and grabbing the new one, no longer applies.  My
profiling says that this saves about 5% of the CPU time for an
all-in-memory seqscan.

src/backend/executor/execTuples.c

@@ -15,7 +15,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execTuples.c,v 1.89 2005/11/22 18:17:10 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execTuples.c,v 1.90 2005/11/25 04:24:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -402,28 +402,38 @@ ExecStoreTuple(HeapTuple tuple,
 	Assert(BufferIsValid(buffer) ? (!shouldFree) : true);
 
 	/*
-	 * clear out any old contents of the slot
+	 * Free any old physical tuple belonging to the slot.
 	 */
-	if (!slot->tts_isempty)
-		ExecClearTuple(slot);
+	if (slot->tts_shouldFree)
+		heap_freetuple(slot->tts_tuple);
 
 	/*
-	 * store the new tuple into the specified slot.
+	 * Store the new tuple into the specified slot.
 	 */
 	slot->tts_isempty = false;
 	slot->tts_shouldFree = shouldFree;
 	slot->tts_tuple = tuple;
 
+	/* Mark extracted state invalid */
+	slot->tts_nvalid = 0;
+
 	/*
 	 * If tuple is on a disk page, keep the page pinned as long as we hold a
 	 * pointer into it.  We assume the caller already has such a pin.
+	 *
+	 * This is coded to optimize the case where the slot previously held a
+	 * tuple on the same disk page: in that case releasing and re-acquiring
+	 * the pin is a waste of cycles.  This is a common situation during
+	 * seqscans, so it's worth troubling over.
 	 */
-	slot->tts_buffer = buffer;
-	if (BufferIsValid(buffer))
-		IncrBufferRefCount(buffer);
-
-	/* Mark extracted state invalid */
-	slot->tts_nvalid = 0;
+	if (slot->tts_buffer != buffer)
+	{
+		if (BufferIsValid(slot->tts_buffer))
+			ReleaseBuffer(slot->tts_buffer);
+		slot->tts_buffer = buffer;
+		if (BufferIsValid(buffer))
+			IncrBufferRefCount(buffer);
+	}
 
 	return slot;
 }

src/backend/executor/nodeBitmapHeapscan.c

@@ -21,7 +21,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeBitmapHeapscan.c,v 1.4 2005/10/15 02:49:17 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeBitmapHeapscan.c,v 1.5 2005/11/25 04:24:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -75,15 +75,6 @@ BitmapHeapNext(BitmapHeapScanState *node)
 	tbm = node->tbm;
 	tbmres = node->tbmres;
 
-	/*
-	 * Clear any reference to the previously returned tuple.  The idea here is
-	 * to not have the tuple slot be the last holder of a pin on that tuple's
-	 * buffer; if it is, we'll need a separate visit to the bufmgr to release
-	 * the buffer.	By clearing here, we get to have the release done by
-	 * ReleaseAndReadBuffer, below.
-	 */
-	ExecClearTuple(slot);
-
 	/*
 	 * Check if we are evaluating PlanQual for tuple of this relation.
 	 * Additional checking is not good, but no other way for now. We could
@@ -94,7 +85,7 @@ BitmapHeapNext(BitmapHeapScanState *node)
 		estate->es_evTuple[scanrelid - 1] != NULL)
 	{
 		if (estate->es_evTupleNull[scanrelid - 1])
-			return slot;		/* return empty slot */
+			return ExecClearTuple(slot);
 
 		ExecStoreTuple(estate->es_evTuple[scanrelid - 1],
 					   slot, InvalidBuffer, false);

src/backend/executor/nodeIndexscan.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeIndexscan.c,v 1.105 2005/11/22 18:17:10 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeIndexscan.c,v 1.106 2005/11/25 04:24:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -74,15 +74,6 @@ IndexNext(IndexScanState *node)
 	slot = node->ss.ss_ScanTupleSlot;
 	scanrelid = ((IndexScan *) node->ss.ps.plan)->scan.scanrelid;
 
-	/*
-	 * Clear any reference to the previously returned tuple.  The idea here is
-	 * to not have the tuple slot be the last holder of a pin on that tuple's
-	 * buffer; if it is, we'll need a separate visit to the bufmgr to release
-	 * the buffer.	By clearing here, we get to have the release done by
-	 * ReleaseAndReadBuffer inside index_getnext.
-	 */
-	ExecClearTuple(slot);
-
 	/*
 	 * Check if we are evaluating PlanQual for tuple of this relation.
 	 * Additional checking is not good, but no other way for now. We could
@@ -93,7 +84,7 @@ IndexNext(IndexScanState *node)
 		estate->es_evTuple[scanrelid - 1] != NULL)
 	{
 		if (estate->es_evTupleNull[scanrelid - 1])
-			return slot;		/* return empty slot */
+			return ExecClearTuple(slot);
 
 		ExecStoreTuple(estate->es_evTuple[scanrelid - 1],
 					   slot, InvalidBuffer, false);

src/backend/executor/nodeSeqscan.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeSeqscan.c,v 1.54 2005/10/15 02:49:17 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeSeqscan.c,v 1.55 2005/11/25 04:24:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -61,15 +61,6 @@ SeqNext(SeqScanState *node)
 	direction = estate->es_direction;
 	slot = node->ss_ScanTupleSlot;
 
-	/*
-	 * Clear any reference to the previously returned tuple.  The idea here is
-	 * to not have the tuple slot be the last holder of a pin on that tuple's
-	 * buffer; if it is, we'll need a separate visit to the bufmgr to release
-	 * the buffer.	By clearing here, we get to have the release done by
-	 * ReleaseAndReadBuffer inside heap_getnext.
-	 */
-	ExecClearTuple(slot);
-
 	/*
 	 * Check if we are evaluating PlanQual for tuple of this relation.
 	 * Additional checking is not good, but no other way for now. We could
@@ -80,7 +71,7 @@ SeqNext(SeqScanState *node)
 		estate->es_evTuple[scanrelid - 1] != NULL)
 	{
 		if (estate->es_evTupleNull[scanrelid - 1])
-			return slot;		/* return empty slot */
+			return ExecClearTuple(slot);
 
 		ExecStoreTuple(estate->es_evTuple[scanrelid - 1],
 					   slot, InvalidBuffer, false);
@@ -93,7 +84,7 @@ SeqNext(SeqScanState *node)
 
 		/* Flag for the next call that no more tuples */
 		estate->es_evTupleNull[scanrelid - 1] = true;
-		return (slot);
+		return slot;
 	}
 
 	/*
@@ -115,6 +106,8 @@ SeqNext(SeqScanState *node)
 					   scandesc->rs_cbuf,		/* buffer associated with this
 												 * tuple */
 					   false);	/* don't pfree this pointer */
+	else
+		ExecClearTuple(slot);
 
 	return slot;
 }

src/backend/executor/nodeTidscan.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeTidscan.c,v 1.43 2005/10/15 02:49:17 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeTidscan.c,v 1.44 2005/11/25 04:24:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -106,13 +106,6 @@ TidNext(TidScanState *node)
 	slot = node->ss.ss_ScanTupleSlot;
 	scanrelid = ((TidScan *) node->ss.ps.plan)->scan.scanrelid;
 
-	/*
-	 * Clear any reference to the previously returned tuple.  This doesn't
-	 * offer any great performance benefit, but it keeps this code in sync
-	 * with SeqNext and IndexNext.
-	 */
-	ExecClearTuple(slot);
-
 	/*
 	 * Check if we are evaluating PlanQual for tuple of this relation.
 	 * Additional checking is not good, but no other way for now. We could
@@ -123,7 +116,7 @@ TidNext(TidScanState *node)
 		estate->es_evTuple[scanrelid - 1] != NULL)
 	{
 		if (estate->es_evTupleNull[scanrelid - 1])
-			return slot;		/* return empty slot */
+			return ExecClearTuple(slot);
 
 		/*
 		 * XXX shouldn't we check here to make sure tuple matches TID list? In
@@ -135,7 +128,7 @@ TidNext(TidScanState *node)
 
 		/* Flag for the next call that no more tuples */
 		estate->es_evTupleNull[scanrelid - 1] = true;
-		return (slot);
+		return slot;
 	}
 
 	/*