ORDER BY index traversal direction in the optimizer trace

mysql-test/main/opt_trace.result

@@ -2176,6 +2176,7 @@ explain  select * from t1 where a=1 and b=2 order by c limit 1	{
                 {
                   "index": "a_a",
                   "can_resolve_order": true,
+                  "direction": 1,
                   "updated_limit": 47,
                   "index_scan_time": 47,
                   "usable": false,
@@ -2184,6 +2185,7 @@ explain  select * from t1 where a=1 and b=2 order by c limit 1	{
                 {
                   "index": "a_c",
                   "can_resolve_order": true,
+                  "direction": 1,
                   "updated_limit": 47,
                   "range_scan_time": 4.331020747,
                   "index_scan_time": 4.331020747,

sql/sql_select.cc

@@ -232,9 +232,7 @@ static bool test_if_cheaper_ordering(const JOIN_TAB *tab,
                                      ha_rows *new_select_limit,
                                      uint *new_used_key_parts= NULL,
                                      uint *saved_best_key_parts= NULL);
-static int test_if_order_by_key(JOIN *join,
-                                ORDER *order, TABLE *table, uint idx,
-				uint *used_key_parts);
+static int test_if_order_by_key(JOIN *, ORDER *, TABLE *, uint, uint *);
 static bool test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,
 				    ha_rows select_limit, bool no_changes,
                                     const key_map *map);
@@ -28967,6 +28965,7 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table,
       DBUG_ASSERT (ref_key != (int) nr);
 
       possible_key.add("can_resolve_order", true);
+      possible_key.add("direction", direction);
       bool is_covering= (table->covering_keys.is_set(nr) ||
                          (table->file->index_flags(nr, 0, 1) &
                           HA_CLUSTERED_INDEX));