presto-orc/src/main/java/io/prestosql/orc/writer/StructColumnWriter.java/StructColumnWriter/finishRowGroup
class StructColumnWriter:    @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);
        structFields.stream()
                .map(ColumnWriter::finishRowGroup)
                .forEach(columnStatistics::putAll);
        return columnStatistics.build();
    }