presto-orc/src/main/java/io/prestosql/orc/writer/MapColumnWriter.java/MapColumnWriter/getNestedColumnWriters
class MapColumnWriter:    @Override
    public List<ColumnWriter> getNestedColumnWriters()
    {
        return ImmutableList.<ColumnWriter>builder()
                .add(keyWriter)
                .addAll(keyWriter.getNestedColumnWriters())
                .add(valueWriter)
                .addAll(valueWriter.getNestedColumnWriters())
                .build();
    }