presto-orc/src/main/java/io/prestosql/orc/writer/SliceDictionaryColumnWriter.java/SliceDictionaryColumnWriter/finishRowGroup
class SliceDictionaryColumnWriter:    @Override
    public Map<OrcColumnId, ColumnStatistics> finishRowGroup()
    {
        checkState(!closed);
        checkState(inRowGroup);
        inRowGroup = false;

        if (directEncoded) {
            return directColumnWriter.finishRowGroup();
        }

        ColumnStatistics statistics = statisticsBuilder.buildColumnStatistics();
        rowGroups.add(new DictionaryRowGroup(values, rowGroupValueCount, statistics));
        rowGroupValueCount = 0;
        statisticsBuilder = newStringStatisticsBuilder();
        values = new IntBigArray();
        return ImmutableMap.of(columnId, statistics);
    }