@Override
  public List<String> getOutputColumnNames() {
    return children.stream()
        .map(PlanNode::getOutputColumnNames)
        .flatMap(List::stream)
        .collect(Collectors.toList());
  }
This node is responsible for merge two or more TsBlocks have been aligned. TsBlocks must have
same time column or no time column at all, and we can merge the value column directly without
compare.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/planner/plan/node/process/HorizontallyConcatNode.java