filesort still select when desc/asc index matches ORDER BY

create table t1 (a int, b int, c int, key r (a desc, b asc));

insert t1 select seq % 10, seq div 10, seq from seq_1_to_55;

insert t1 values (NULL, NULL, NULL), (9, NULL, NULL);

MariaDB [test]> explain format=json select * from t1 order by a desc,b asc\G

*************************** 1. row ***************************

EXPLAIN: {

  "query_block": {

    "select_id": 1,

    "nested_loop": [

      {

        "read_sorted_file": {

          "filesort": {

            "sort_key": "t1.a desc, t1.b",

            "table": {

              "table_name": "t1",

              "access_type": "ALL",

              "rows": 57,

              "filtered": 100

            }

          }

        }

      }

    ]

  }

}

MariaDB [test]> explain format=json select * from t1 order by a asc,b desc\G

*************************** 1. row ***************************

EXPLAIN: {

  "query_block": {

    "select_id": 1,

    "nested_loop": [

      {

        "read_sorted_file": {

          "filesort": {

            "sort_key": "t1.a, t1.b desc",

            "table": {

              "table_name": "t1",

              "access_type": "ALL",

              "rows": 57,

              "filtered": 100

            }

          }

        }

      }

    ]

  }

}