presto-orc/src/main/java/io/prestosql/orc/writer/SliceDictionaryColumnWriter.java/SliceDictionaryColumnWriter/beginRowGroup
class SliceDictionaryColumnWriter:    @Override
    public void beginRowGroup()
    {
        checkState(!inRowGroup);
        inRowGroup = true;

        if (directEncoded) {
            directColumnWriter.beginRowGroup();
        }
    }