private Operator generateOnlyChildOperator(PlanNode node, LocalExecutionPlanContext context) {
    List<Operator> children =
        node.getChildren().stream()
            .map(child -> child.accept(this, context))
            .collect(Collectors.toList());
    Validate.isTrue(children.size() == 1);
    return children.get(0);
  }
This Visitor is responsible for transferring PlanNode Tree to Operator Tree.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/planner/OperatorTreeGenerator.java