presto-main/src/main/java/io/prestosql/sql/planner/optimizations/HashGenerationOptimizer.java/HashGenerationOptimizer/HashComputation
class HashGenerationOptimizer:        private HashComputation(Iterable<Symbol> fields)
        {
            requireNonNull(fields, "fields is null");
            this.fields = ImmutableList.copyOf(fields);
            checkArgument(!this.fields.isEmpty(), "fields can not be empty");
        }