presto-main/src/main/java/io/prestosql/sql/planner/optimizations/HashGenerationOptimizer.java/Rewriter/visitEnforceSingleRow
class Rewriter:        @Override
        public PlanWithProperties visitEnforceSingleRow(EnforceSingleRowNode node, HashComputationSet parentPreference)
        {
            // this plan node can only have a single input symbol, so do not add extra hash symbols
            return planSimpleNodeWithProperties(node, new HashComputationSet(), true);
        }