@Override
  public List<FragmentInstance> parallelPlan() {
    prepare();
    calculateNodeTopologyBetweenInstance();
    return fragmentInstanceList;
  }
A simple implementation of IFragmentParallelPlaner. This planner will transform one PlanFragment
into only one FragmentInstance.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/planner/distribution/SimpleFragmentParallelPlanner.java