Fixes:

I found another bug in btree index.  Looking at the code it seems that NULL
keys are never used to build or scan a btree index (see the explain commands
in the example).  However this is not the case when a null key is retrieved
in an outer loop of a join select and used in an index scan of an inner loop.
This bug causes at least three kinds of problems:

1)  the backend crashes when it tries to compare a text string with a null.

2)  it is not possible to find tuples with null keys in a join.

3)  null is considered equal to 0 when the datum is passed by value, see
    the last query.

Submitted by: Massimo Dal Zotto <dz@cs.unitn.it>

src/backend/access/common/heapvalid.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *    $Header: /cvsroot/pgsql/src/backend/access/common/Attic/heapvalid.c,v 1.6 1996/10/21 11:49:36 scrappy Exp $
+ *    $Header: /cvsroot/pgsql/src/backend/access/common/Attic/heapvalid.c,v 1.7 1996/10/30 06:07:56 scrappy Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -71,6 +71,10 @@ heap_keytest(HeapTuple t,
 	    /* XXX eventually should check if SK_ISNULL */
 	    return false;
 	
+	if (keys->sk_flags & SK_ISNULL) {
+	    return (false);
+	}
+
 	if (keys->sk_flags & SK_COMMUTE)
 	    test = (long) FMGR_PTR2(keys->sk_func, keys->sk_procedure,
 				    keys->sk_argument, atp);

src/backend/access/common/indexvalid.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *    $Header: /cvsroot/pgsql/src/backend/access/common/Attic/indexvalid.c,v 1.6 1996/10/21 11:49:38 scrappy Exp $
+ *    $Header: /cvsroot/pgsql/src/backend/access/common/Attic/indexvalid.c,v 1.7 1996/10/30 06:07:55 scrappy Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -68,6 +68,10 @@ index_keytest(IndexTuple tuple,
 	    return (false);
 	}
 	
+	if (key[0].sk_flags & SK_ISNULL) {
+	    return (false);
+	}
+
 	if (key[0].sk_flags & SK_COMMUTE) {
 	    test = (int) (*(key[0].sk_func))
 		(DatumGetPointer(key[0].sk_argument),

src/backend/access/nbtree/nbtsearch.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *    $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.4 1996/10/23 07:39:10 scrappy Exp $
+ *    $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.5 1996/10/30 06:08:01 scrappy Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -269,6 +269,18 @@ _bt_skeycmp(Relation rel,
 				  &isNull);
 	keyDatum  = entry->sk_argument;
 	
+	/*
+	 * This may happen in a nested loop if an attribute used
+	 * as scan key is null.			DZ 29-10-1996
+	 */
+	if ((entry->sk_flags & SK_ISNULL) || (isNull)) {
+	    if ((entry->sk_flags & SK_ISNULL) && (isNull)) {
+		return (true);
+	    } else {
+		return (false);
+	    }
+	}
+
 	compare = _bt_invokestrat(rel, i, strat, keyDatum, attrDatum);
 	if (!compare)
 	    return (false);
@@ -501,6 +513,19 @@ _bt_compare(Relation rel,
 	entry = &scankey[i - 1];
 	attno = entry->sk_attno;
 	datum = index_getattr(itup, attno, itupdesc, &null);
+
+	/*
+	 * This may happen in a nested loop if an attribute used
+	 * as scan key is null.			DZ 29-10-1996
+	 */
+	if ((entry->sk_flags & SK_ISNULL) || (null)) {
+	    if ((entry->sk_flags & SK_ISNULL) && (null)) {
+		return (0);
+	    } else {
+		return (null ? +1 : -1);
+	    }
+	}
+
 	tmpres = (long) FMGR_PTR2(entry->sk_func, entry->sk_procedure,
 				  entry->sk_argument, datum);
 	result = tmpres;
@@ -641,7 +666,7 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
      *	       hardwired attno == 1.
      */
     proc = index_getprocid(rel, 1, BTORDER_PROC);
-    ScanKeyEntryInitialize(&skdata, 0x0, 1, proc,
+    ScanKeyEntryInitialize(&skdata, so->keyData[0].sk_flags, 1, proc,
 			   so->keyData[0].sk_argument);
     
     stack = _bt_search(rel, 1, &skdata, &buf);

src/backend/executor/nodeIndexscan.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *    $Header: /cvsroot/pgsql/src/backend/executor/nodeIndexscan.c,v 1.1.1.1 1996/07/09 06:21:26 scrappy Exp $
+ *    $Header: /cvsroot/pgsql/src/backend/executor/nodeIndexscan.c,v 1.2 1996/10/30 06:08:10 scrappy Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -273,6 +273,11 @@ ExecIndexReScan(IndexScan *node, ExprContext *exprCtxt, Plan* parent)
 		scanvalue = (Datum)
 		    ExecEvalExpr(scanexpr, exprCtxt, &isNull, &isDone);
 		scan_keys[j].sk_argument = scanvalue;
+		if (isNull) {
+		    scan_keys[j].sk_flags |= SK_ISNULL;
+		} else {
+		    scan_keys[j].sk_flags &= ~SK_ISNULL;
+		}
 	    }
 	}
     }