presto-main/src/main/java/io/prestosql/sql/planner/optimizations/AddLocalExchanges.java/Rewriter/visitPlan
class Rewriter:        @Override
        protected PlanWithProperties visitPlan(PlanNode node, StreamPreferredProperties parentPreferences)
        {
            return planAndEnforceChildren(
                    node,
                    parentPreferences.withoutPreference().withDefaultParallelism(session),
                    parentPreferences.withDefaultParallelism(session));
        }