@Override
  public TimeRange nextTimeRange() {
    if (hasCachedTimeRange || hasNextTimeRange()) {
      hasCachedTimeRange = false;
      return curTimeRange;
    }
    return null;
  }
Used for aggregation with only one time window. i.e. Aggregation without group by.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/aggregation/timerangeiterator/SingleTimeWindowIterator.java