@Override
  public String toString() {
    return String.format(
        "Query[%s]-[%s-%s-SourceHandle-%s]",
        localFragmentInstanceId.getQueryId(),
        localFragmentInstanceId.getFragmentId(),
        fullFragmentInstanceId,
        localPlanNodeId);
  }
this is set to true after calling isBlocked() at least once which indicates that this
SourceHandle needs to output data.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/exchange/source/SourceHandle.java