@Override
    public int compare(DriverTask o1, DriverTask o2) {
      if (o1.getDriverTaskId().equals(o2.getDriverTaskId())) {
        return 0;
      }
      if (o1.getDDL() < o2.getDDL()) {
        return -1;
      }
      if (o1.getDDL() > o2.getDDL()) {
        return 1;
      }
      return o1.getDriverTaskId().compareTo(o2.getDriverTaskId());
    }
a comparator of ddl, the less the ddl is, the low order it has.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/schedule/task/DriverTask.java