@Override
  public TimeRange getFirstTimeRange() {
    if (isAscending) {
      return getLeftmostTimeRange();
    } else {
      return getRightmostTimeRange();
    }
  }
This class iteratively generates pre-aggregated time windows.

<p>For example, startTime = 0, endTime = 11, interval = 5, slidingStep = 3, return
[0,2),[2,3),[3,5),[5,6),[6,8),[8,9),[9,10)
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/aggregation/timerangeiterator/PreAggrWindowIterator.java