@Override
  public long getBufferRetainedSizeInBytes() {
    return bufferRetainedSizeInBytes;
  }
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