You could have used

explain format=json

which would have given you more info something like this

EXPLAIN: {
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "16.72"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "departments",
          <skipped>
      },
      {
        "table": {
          "table_name": "<subquery2>",
          "access_type": "eq_ref",
          "key": "<auto_key>",
          "key_length": "4",
          "ref": [
            "employees.departments.dept_no"
          ],
          "rows_examined_per_scan": 1,
          "materialized_from_subquery": {
            "using_temporary_table": true,
            "query_block": {
              "table": {
                "table_name": "dept_manager",
                "access_type": "ALL",
                "possible_keys": [
                  "dept_no"
                ],
                "rows_examined_per_scan": 24,
                "rows_produced_per_join": 21,
                "filtered": "90.00",
                "cost_info": {
                  "read_cost": "1.48",
                  "eval_cost": "4.32",
                  "prefix_cost": "5.80",
                  "data_read_per_join": "345"
                },
                "used_columns": [
                  "dept_no",
                  "to_date"
                ],
                "attached_condition": "(`employees`.`dept_manager`.`to_date` is not null)"
              }
            }
          }
        }
      }
    ]
  }
}

This way few things could have been explain better.