@Override
  public long currentOutputTime() {
    // display time as 0 in aggregation result
    return curTimeRange.getMin() == Long.MIN_VALUE ? 0 : curTimeRange.getMin();
  }
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