presto-main/src/main/java/io/prestosql/sql/planner/PlanFragmenter.java/PlanFragmenter/buildRootFragment
class PlanFragmenter:        public SubPlan buildRootFragment(PlanNode root, FragmentProperties properties)
        {
            return buildFragment(root, properties, new PlanFragmentId(String.valueOf(ROOT_FRAGMENT_ID)));
        }