@Override
  public boolean hasNext() throws Exception {
    return !sourceHandle.isFinished();
  }
For ExchangeOperator in pipeline, the maxReturnSize is equal to the maxReturnSize of the child
operator.

@param maxReturnSize max return size of child operator
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/source/ExchangeOperator.java