presto-main/src/main/java/io/prestosql/sql/planner/optimizations/DistinctOutputQueryUtil.java/IsDistinctPlanVisitor/visitEnforceSingleRow
class IsDistinctPlanVisitor:        @Override
        public Boolean visitEnforceSingleRow(EnforceSingleRowNode node, Void context)
        {
            return true;
        }