presto-orc/src/main/java/io/prestosql/orc/metadata/statistics/StringStatisticsBuilder.java/StringStatisticsBuilder/buildStringStatistics
class StringStatisticsBuilder:    private Optional<StringStatistics> buildStringStatistics()
    {
        if (nonNullValueCount == 0) {
            return Optional.empty();
        }
        minimum = dropStringMinMaxIfNecessary(minimum);
        maximum = dropStringMinMaxIfNecessary(maximum);
        if (minimum == null && maximum == null) {
            // Create string stats only when min or max is not null.
            // This corresponds to the behavior of metadata reader.
            return Optional.empty();
        }
        return Optional.of(new StringStatistics(minimum, maximum, sum));
    }