@Override
  public boolean isFinished() throws Exception {
    // return true if any child is finished.
    for (int i = 0; i < inputOperatorsCount; i++) {
      if (isEmpty(i) && children.get(i).isFinished()) {
        return true;
      }
    }
    // return false if all children still hava data
    return false;
  }
Try to cache one result of each child.

@return true if results of all children are ready. Return false if some children is blocked or
    return null.
@throws Exception errors happened while getting tsblock from children
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/process/join/InnerTimeJoinOperator.java