@Override
    public RelNode defaultMethod(QueryOperation other) {
        other.getChildren().forEach(child -> relBuilder.push(child.accept(this)));
        return other.accept(singleRelVisitor);
    }
Converter from Flink's specific relational representation: {@link QueryOperation} to Calcite's specific relational
representation: {@link RelNode}.
core/src/main/java/org/apache/flink/table/planner/plan/QueryOperationConverter.java