presto-main/src/main/java/io/prestosql/sql/planner/optimizations/HashGenerationOptimizer.java/HashGenerationOptimizer/computeHash
class HashGenerationOptimizer:    private static Optional<HashComputation> computeHash(Iterable<Symbol> fields)
    {
        requireNonNull(fields, "fields is null");
        List<Symbol> symbols = ImmutableList.copyOf(fields);
        if (symbols.isEmpty()) {
            return Optional.empty();
        }
        return Optional.of(new HashComputation(fields));
    }