presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/GraphvizPrinter/getNodeId
class GraphvizPrinter:        public String getNodeId(PlanNode from)
        {
            int nodeId;

            if (planNodeIds.containsKey(from)) {
                nodeId = planNodeIds.get(from);
            }
            else {
                idCount++;
                planNodeIds.put(from, idCount);
                nodeId = idCount;
            }
            return ("plannode_" + nodeId);
        }