presto-main/src/main/java/io/prestosql/sql/planner/PlanFragmenter.java/Fragmenter/buildSubPlan
class Fragmenter:        private SubPlan buildSubPlan(PlanNode node, FragmentProperties properties, RewriteContext<FragmentProperties> context)
        {
            PlanFragmentId planFragmentId = nextFragmentId();
            PlanNode child = context.rewrite(node, properties);
            return buildFragment(child, properties, planFragmentId);
        }