presto-main/src/main/java/io/prestosql/sql/planner/optimizations/HashGenerationOptimizer.java/HashGenerationOptimizer/visitPlan
class HashGenerationOptimizer:        @Override
        protected PlanWithProperties visitPlan(PlanNode node, HashComputationSet parentPreference)
        {
            return planSimpleNodeWithProperties(node, parentPreference);
        }