protected PriorityMergeReader getPriorityMergeReader() {
    return new PriorityMergeReader();
  }
Initialize the query data source. This method should be called <b>before any other methods</b>.

@param dataSource the query data source
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/source/SeriesScanUtil.java