presto-main/src/main/java/io/prestosql/execution/scheduler/PhasedExecutionSchedule.java/Visitor/addEdges
class Visitor:        private void addEdges(Set<PlanFragmentId> sourceFragments, Set<PlanFragmentId> targetFragments)
        {
            for (PlanFragmentId targetFragment : targetFragments) {
                for (PlanFragmentId sourceFragment : sourceFragments) {
                    graph.addEdge(sourceFragment, targetFragment);
                }
            }
        }