presto-main/src/main/java/io/prestosql/sql/planner/planprinter/PlanPrinter.java/PlanPrinter/visitTopN
class PlanPrinter:        @Override
        public Void visitTopN(TopNNode node, Void context)
        {
            String keys = node.getOrderingScheme()
                    .getOrderBy().stream()
                    .map(input -> input + " " + node.getOrderingScheme().getOrdering(input))
                    .collect(joining(", "));

            addNode(node,
                    format("TopN%s", node.getStep() == TopNNode.Step.PARTIAL ? "Partial" : ""),
                    format("[%s by (%s)]", node.getCount(), keys));
            return processChildren(node, context);
        }