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

        ColumnStatistics statistics = new ColumnStatistics((long) nonNullValueCount, 0, null, null, null, null, null, null, null, null);
        rowGroupColumnStatistics.add(statistics);
        nonNullValueCount = 0;

        ImmutableMap.Builder<OrcColumnId, ColumnStatistics> columnStatistics = ImmutableMap.builder();
        columnStatistics.put(columnId, statistics);
        columnStatistics.putAll(keyWriter.finishRowGroup());
        columnStatistics.putAll(valueWriter.finishRowGroup());
        return columnStatistics.build();
    }