Bug #42760: Select doesn't return desired results when we have null values

Part 2 : 
There was a special optimization on the ref access method for 
ORDER BY ... DESC that was set without actually looking on the type of the 
selected index for ORDER BY.
Fixed the SELECT ... ORDER BY .. DESC (it uses a different code path compared
to the ASC that has been fixed with the previous fix).

mysql-test/r/order_by.result

@@ -1111,5 +1111,14 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;
 col
 1
+# Must use ref-or-null on the a_c index
+EXPLAIN
+SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c DESC;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+x	x	x	ref_or_null	a_c,a	x	x	x	x	x
+# Must return 1 row
+SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c DESC;
+col
+1
 DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/order_by.test

@@ -778,6 +778,15 @@ SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;
 --echo # Must return 1 row
 SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;
 
+# part 2 of the problem : DESC test cases
+--echo # Must use ref-or-null on the a_c index
+--replace_column 1 x 2 x 3 x 6 x 7 x 8 x 9 x 10 x
+EXPLAIN
+SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c DESC;
+--echo # Must return 1 row
+SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c DESC;
+
+
 DROP TABLE t1;
 
 

sql/sql_select.cc

@@ -12824,7 +12824,7 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
 	  }
 	  DBUG_RETURN(1);
 	}
-	if (tab->ref.key_parts <= used_key_parts)
+	if (tab->ref.key_parts <= used_key_parts && tab->type == JT_REF)
 	{
 	  /*
 	    SELECT * FROM t1 WHERE a=1 ORDER BY a DESC,b DESC