presto-orc/src/main/java/io/prestosql/orc/writer/SliceDictionaryColumnWriter.java/SliceDictionaryColumnWriter/getColumnStripeStatistics
class SliceDictionaryColumnWriter:    @Override
    public Map<OrcColumnId, ColumnStatistics> getColumnStripeStatistics()
    {
        checkState(closed);
        if (directEncoded) {
            return directColumnWriter.getColumnStripeStatistics();
        }

        return ImmutableMap.of(columnId, ColumnStatistics.mergeColumnStatistics(rowGroups.stream()
                .map(DictionaryRowGroup::getColumnStatistics)
                .collect(toList())));
    }