presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/GraphvizPrinter/printSubPlan
class GraphvizPrinter:    private static void printSubPlan(SubPlan plan, Map<PlanFragmentId, PlanFragment> fragmentsById, PlanNodeIdGenerator idGenerator, StringBuilder output)
    {
        PlanFragment fragment = plan.getFragment();
        printFragmentNodes(output, fragment, idGenerator);
        fragment.getRoot().accept(new EdgePrinter(output, fragmentsById, idGenerator), null);

        for (SubPlan child : plan.getChildren()) {
            printSubPlan(child, fragmentsById, idGenerator, output);
        }
    }