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

            printNode(node, format("TopN[%s]", node.getCount()), keys, NODE_COLORS.get(NodeType.TOPN));
            return node.getSource().accept(this, context);
        }