There are some bugs about backward scanning using

indexes.

1. Index Scan using plural indexids never scan backward
   as to the order of indexids.
2. The cursor using Index scan is not usable after moving
   past the end.

This patch solves above bugs.
Moreover the change of _bt_first() would be useful to extend
ORDER BY patch by Jan Wieck for all descending order cases.

Hiroshi Inoue

src/backend/access/nbtree/nbtsearch.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.42 1999/03/28 20:31:58 vadim Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.43 1999/04/13 17:18:28 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -733,7 +733,8 @@ _bt_next(IndexScanDesc scan, ScanDirection dir)
 			return res;
 		}
 
-	} while (keysok >= so->numberOfFirstKeys);
+	} while (keysok >= so->numberOfFirstKeys || 
+		 (keysok == -1 && ScanDirectionIsBackward(dir)));
 
 	ItemPointerSetInvalid(current);
 	so->btso_curbuf = InvalidBuffer;
@@ -775,6 +776,8 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 	BTScanOpaque so;
 	ScanKeyData skdata;
 	Size		keysok;
+	int		i;
+	int		nKeyIndex = -1;
 
 	rel = scan->relation;
 	so = (BTScanOpaque) scan->opaque;
@@ -790,11 +793,34 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 	{
 		_bt_orderkeys(rel, so);
 
-		strat = _bt_getstrat(rel, 1, so->keyData[0].sk_procedure);
+		if (ScanDirectionIsBackward(dir))
+		{
+			for (i=0; i<so->numberOfKeys; i++)
+			{
+				if (so->keyData[i].sk_attno != 1)
+					break;
+				strat = _bt_getstrat(rel, so->keyData[i].sk_attno, 
+					so->keyData[i].sk_procedure);
+				if (strat == BTLessStrategyNumber ||
+				    strat == BTLessEqualStrategyNumber||
+				    strat == BTEqualStrategyNumber)
+				{
+					nKeyIndex = i;
+					break;
+				}
+			}
+		}
+		else 
+		{
+			strat = _bt_getstrat(rel, 1, so->keyData[0].sk_procedure);
 
-		/* NOTE: it assumes ForwardScanDirection */
-		if (strat == BTLessStrategyNumber ||
-			strat == BTLessEqualStrategyNumber)
+			if (strat == BTLessStrategyNumber ||
+			    strat == BTLessEqualStrategyNumber)
+				;
+			else
+				nKeyIndex = 0;
+		}
+		if (nKeyIndex < 0)
 			scan->scanFromEnd = true;
 	}
 	else
@@ -823,8 +849,8 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 		return (RetrieveIndexResult) NULL;
 	}
 	proc = index_getprocid(rel, 1, BTORDER_PROC);
-	ScanKeyEntryInitialize(&skdata, so->keyData[0].sk_flags, 1, proc,
-						   so->keyData[0].sk_argument);
+	ScanKeyEntryInitialize(&skdata, so->keyData[nKeyIndex].sk_flags,
+				1, proc, so->keyData[nKeyIndex].sk_argument);
 
 	stack = _bt_search(rel, 1, &skdata, &buf);
 	_bt_freestack(stack);
@@ -897,7 +923,7 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 
 	/* it's yet other place to add some code latter for is(not)null */
 
-	strat = _bt_getstrat(rel, 1, so->keyData[0].sk_procedure);
+	strat = _bt_getstrat(rel, 1, so->keyData[nKeyIndex].sk_procedure);
 
 	switch (strat)
 	{
@@ -914,9 +940,6 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 					result = _bt_compare(rel, itupdesc, page, 1, &skdata, offnum);
 				} while (result <= 0);
 
-				/* if this is true, the key we just looked at is gone */
-				if (result > 0)
-					_bt_twostep(scan, &buf, ForwardScanDirection);
 			}
 			break;
 
@@ -946,6 +969,21 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 				ItemPointerSetInvalid(&(scan->currentItemData));
 				return (RetrieveIndexResult) NULL;
 			}
+			else if (ScanDirectionIsBackward(dir))
+			{
+				do
+				{
+					if (!_bt_twostep(scan, &buf, ForwardScanDirection))
+						break;
+
+					offnum = ItemPointerGetOffsetNumber(current);
+					page = BufferGetPage(buf);
+					result = _bt_compare(rel, itupdesc, page, 1, &skdata, offnum);
+				} while (result == 0);
+
+				if (result < 0)
+					_bt_twostep(scan, &buf, BackwardScanDirection);
+			}
 			break;
 
 		case BTGreaterEqualStrategyNumber:
@@ -1026,6 +1064,11 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 		so->btso_curbuf = buf;
 		return _bt_next(scan, dir);
 	}
+	else if (keysok == -1 && ScanDirectionIsBackward(dir))
+	{
+		so->btso_curbuf = buf;
+		return _bt_next(scan, dir);
+	}
 	else
 	{
 		ItemPointerSetInvalid(current);
@@ -1495,6 +1538,11 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
 		so->btso_curbuf = buf;
 		return _bt_next(scan, dir);
 	}
+	else if (keysok == -1 && ScanDirectionIsBackward(dir))
+	{
+		so->btso_curbuf = buf;
+		return _bt_next(scan, dir);
+	}
 	else
 	{
 		ItemPointerSetInvalid(current);

src/backend/access/nbtree/nbtutils.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtutils.c,v 1.25 1999/02/13 23:14:37 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtutils.c,v 1.26 1999/04/13 17:18:29 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -367,8 +367,14 @@ _bt_checkkeys(IndexScanDesc scan, IndexTuple tuple, Size *keysok)
 							  &isNull);
 
 		/* btree doesn't support 'A is null' clauses, yet */
-		if (isNull || key[0].sk_flags & SK_ISNULL)
+		if (key[0].sk_flags & SK_ISNULL)
 			return false;
+		if (isNull)
+		{
+			if (*keysok < so->numberOfFirstKeys)
+				*keysok = -1;
+			return false;
+		}
 
 		if (key[0].sk_flags & SK_COMMUTE)
 		{

src/backend/executor/nodeIndexscan.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/executor/nodeIndexscan.c,v 1.33 1999/02/21 03:48:40 scrappy Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/executor/nodeIndexscan.c,v 1.34 1999/04/13 17:18:29 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -96,6 +96,8 @@ IndexNext(IndexScan *node)
 	Buffer		buffer = InvalidBuffer;
 	int			numIndices;
 
+ 	bool		bBackward;
+ 	int		indexNumber;
 	/* ----------------
 	 *	extract necessary information from index scan node
 	 * ----------------
@@ -151,8 +153,26 @@ IndexNext(IndexScan *node)
 	 *	appropriate heap tuple.. else return NULL.
 	 * ----------------
 	 */
-	while (indexstate->iss_IndexPtr < numIndices)
-	{
+ 	bBackward = ScanDirectionIsBackward(direction);
+ 	if (bBackward)
+  	{
+ 		indexNumber = numIndices - indexstate->iss_IndexPtr - 1;
+ 		if (indexNumber < 0)
+ 		{
+ 			indexNumber = 0;
+ 			indexstate->iss_IndexPtr = numIndices - 1;
+ 		}
+ 	}
+ 	else
+ 	{
+ 		if ((indexNumber = indexstate->iss_IndexPtr) < 0)
+ 		{
+ 			indexNumber = 0;
+ 			indexstate->iss_IndexPtr = 0;
+ 		}
+ 	}
+ 	while (indexNumber < numIndices)
+ 	{
 		scandesc = scanDescs[indexstate->iss_IndexPtr];
 		while ((result = index_getnext(scandesc, direction)) != NULL)
 		{
@@ -204,8 +224,14 @@ IndexNext(IndexScan *node)
 			if (BufferIsValid(buffer))
 				ReleaseBuffer(buffer);
 		}
-		if (indexstate->iss_IndexPtr < numIndices)
-			indexstate->iss_IndexPtr++;
+ 		if (indexNumber < numIndices)
+ 		{
+ 			indexNumber++;
+ 			if (bBackward)
+ 				indexstate->iss_IndexPtr--;
+ 			else
+ 				indexstate->iss_IndexPtr++;
+ 		}
 	}
 	/* ----------------
 	 *	if we get here it means the index scan failed so we
@@ -294,7 +320,7 @@ ExecIndexReScan(IndexScan *node, ExprContext *exprCtxt, Plan *parent)
 	runtimeKeyInfo = (Pointer *) indexstate->iss_RuntimeKeyInfo;
 	indxqual = node->indxqual;
 	numScanKeys = indexstate->iss_NumScanKeys;
-	indexstate->iss_IndexPtr = 0;
+	indexstate->iss_IndexPtr = -1;
 
 	/* If this is re-scanning of PlanQual ... */
 	if (estate->es_evTuple != NULL && 
@@ -611,7 +637,7 @@ ExecInitIndexScan(IndexScan *node, EState *estate, Plan *parent)
 	  */
 	indexstate = makeNode(IndexScanState);
 	indexstate->iss_NumIndices = 0;
-	indexstate->iss_IndexPtr = 0;
+	indexstate->iss_IndexPtr = -1;
 	indexstate->iss_ScanKeys = NULL;
 	indexstate->iss_NumScanKeys = NULL;
 	indexstate->iss_RuntimeKeyInfo = NULL;
@@ -635,7 +661,7 @@ ExecInitIndexScan(IndexScan *node, EState *estate, Plan *parent)
 	indxid = node->indxid;
 	indxqual = node->indxqual;
 	numIndices = length(indxid);
-	indexPtr = 0;
+	indexPtr = -1;
 
 	CXT1_printf("ExecInitIndexScan: context is %d\n", CurrentMemoryContext);