diff -r 3f065ec72ab8 pgbulkload/lib/pg_btree.c
--- a/pgbulkload/lib/pg_btree.c	Fri Jan 20 09:26:20 2012 -0600
+++ b/pgbulkload/lib/pg_btree.c	Wed Jan 25 13:37:43 2012 -0600
@@ -398,6 +398,8 @@
 	BTReaderTerm(&reader);
 }
 
+void merge_tuples(Relation heap, IndexTuple itup_dst, IndexTuple itup_src);
+
 /*
  * _bt_mergeload - Merge two streams of index tuples into new index files.
  */
@@ -462,7 +464,6 @@
 				}
 				else
 				{
-					// TODO -- BSJ
 					if (on_duplicate == ON_DUPLICATE_KEEP_NEW)
 					{
 						self->dup_old++;
@@ -470,7 +471,21 @@
 							RelationGetRelationName(wstate->index));
 						itup2 = BTReaderGetNextItem(btspool2);
 					}
-					else 
+					else if (on_duplicate == ON_DUPLICATE_MERGE)
+					{
+						self->dup_old++;
+
+						// merge from itup into itup2 where itup's col[i] is not null
+						// but itup2's col[i] IS null
+						merge_tuples(heapRel, itup2, itup);				
+
+						ItemPointerCopy(&t_tid2, &itup2->t_tid);
+						self->dup_new++;
+						remove_duplicate(self, heapRel, itup,
+							RelationGetRelationName(wstate->index));
+						itup = BTSpoolGetNextItem(btspool, itup, &should_free);
+					}
+					else
 					{
 						ItemPointerCopy(&t_tid2, &itup2->t_tid);
 						self->dup_new++;
@@ -950,6 +965,113 @@
 		self->dup_old + self->dup_new, relname);
 }
 
+// returns Buffer after locking it (BUFFER_LOCK_SHARE then BUFFER_LOCK_UNLOCK)
+Buffer load_buffer(Relation heap, IndexTuple itup, HeapTupleData *tuple /*OUT */, ItemId *itemid /*OUT */ )
+{
+	BlockNumber		blknum;
+	BlockNumber		offnum;
+	Buffer			buffer;
+	Page			page;
+
+	blknum = ItemPointerGetBlockNumber(&itup->t_tid);
+	offnum = ItemPointerGetOffsetNumber(&itup->t_tid);
+	buffer = ReadBuffer(heap, blknum);
+
+	LockBuffer(buffer, BUFFER_LOCK_SHARE);
+	page = BufferGetPage(buffer);
+	*itemid = PageGetItemId(page, offnum);
+	tuple->t_data = ItemIdIsNormal(*itemid)
+		? (HeapTupleHeader) PageGetItem(page, *itemid)
+		: NULL;
+	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+	return buffer;
+}
+
+void load_tuple(Relation		heap, 
+				HeapTuple		tuple, 
+				IndexTuple		itup, 
+				ItemId			itemid,
+				TupleDesc *		tupdesc		/* OUT */,				
+				int *			ncolumns	/* OUT */, 
+				Datum **		values		/* OUT */, 
+				bool **			nulls		/* OUT */)
+{
+	*tupdesc = RelationGetDescr(heap);
+
+	tuple->t_len = ItemIdGetLength(itemid);
+	tuple->t_self = itup->t_tid;
+
+	*ncolumns = (*tupdesc)->natts;
+	*values = (Datum *) palloc(*ncolumns * sizeof(Datum));
+	*nulls = (bool *) palloc(*ncolumns * sizeof(bool));
+
+	/* Break down the tuple into fields */
+	heap_deform_tuple(tuple, *tupdesc, *values, *nulls);
+}
+
+void merge_tuples(Relation heap, IndexTuple itup_dst, IndexTuple itup_src)
+{
+	HeapTupleData	tuple_src;
+	HeapTupleData	tuple_dst;
+	Buffer			buffer_src;
+	Buffer			buffer_dst;
+	ItemId			itemid_src;
+	ItemId			itemid_dst;
+
+	// load buffers
+	buffer_src = load_buffer(heap, itup_src, &tuple_src, &itemid_src);
+	buffer_dst = load_buffer(heap, itup_dst, &tuple_dst, &itemid_dst);
+
+	if (tuple_src.t_data != NULL)
+	{
+		int			ncolumns_src, ncolumns_dst;
+		int			i;
+		Datum		*values_src = NULL, *values_dst = NULL;
+		TupleDesc	tupdesc_dst, tupdesc_src;
+		bool		*nulls_src = NULL, *nulls_dst = NULL;				
+		bool		* do_replace = NULL;
+		bool		tuple_updated = false;
+		
+		// load source
+		load_tuple(heap, &tuple_src, itup_src, itemid_src, &tupdesc_src, &ncolumns_src, &values_src, &nulls_src);
+		
+		// load destination
+		load_tuple(heap, &tuple_dst, itup_dst, itemid_dst, &tupdesc_dst, &ncolumns_dst, &values_dst, &nulls_dst);
+
+		do_replace = (bool *) palloc(ncolumns_dst * sizeof(bool));
+
+		for (i = 0; i < ncolumns_dst && i < ncolumns_src; ++i)
+		{		
+			do_replace[i] = false;
+
+			if (nulls_dst[i] && !nulls_src[i])
+			{
+				values_dst[i] = values_src[i];
+				nulls_dst[i] = nulls_src[i];
+				do_replace[i] = true;
+
+				// update new row
+				tuple_updated = true;
+			}
+		}
+		
+		if (tuple_updated)
+		{
+			HeapTuple new_tuple = heap_modify_tuple(&tuple_dst, tupdesc_dst, values_dst, nulls_dst, do_replace);			
+			simple_heap_update(heap, &(tuple_dst.t_self), new_tuple);
+		}
+
+		pfree(do_replace);
+		pfree(values_src);
+		pfree(nulls_src);
+		pfree(values_dst);
+		pfree(nulls_dst);
+	}
+
+	ReleaseBuffer(buffer_src);
+	ReleaseBuffer(buffer_dst);
+}
+
 char *
 tuple_to_cstring(TupleDesc tupdesc, HeapTuple tuple)
 {