presto-main/src/main/java/io/prestosql/connector/system/jdbc/FilterUtil.java/FilterUtil/stringFilter
class FilterUtil:    public static Optional<String> stringFilter(TupleDomain<Integer> constraint, int index)
    {
        if (constraint.isNone()) {
            return Optional.empty();
        }

        Domain domain = constraint.getDomains().get().get(index);
        if ((domain == null) || !domain.isSingleValue()) {
            return Optional.empty();
        }

        Object value = domain.getSingleValue();
        if (value instanceof Slice) {
            return Optional.of(((Slice) value).toStringUtf8());
        }
        return Optional.empty();
    }