Window functions: two functions, one (wrong) sorting is used

create table t1 (

  a int,

  b int,

  c int

);

insert into t1 values 

 (10, 1, 1),

 (10, 3, 10),

 (10, 1, 10),

 (10, 3, 100),

 (10, 5, 1000),

 (10, 1, 100);

explain format=json

select 

  a,b,c,

  row_number() over (partition by a),

  row_number() over (partition by a, b)

from t1;

  "query_block": {

    "select_id": 1,

    "window_functions_computation": {

      "sorts": {

        "filesort": {

          "sort_key": "t1.a"

        }

      },

      "temporary_table": {

        "table": {

          "table_name": "t1",

          "access_type": "ALL",

          "rows": 6,

          "filtered": 100

        }

      }

    }

  }