presto-spi/src/main/java/io/prestosql/spi/block/DictionaryBlock.java/DictionaryBlock/getRegion
class DictionaryBlock:    @Override
    public Block getRegion(int positionOffset, int length)
    {
        checkValidRegion(positionCount, positionOffset, length);
        return new DictionaryBlock(idsOffset + positionOffset, length, dictionary, ids, false, dictionarySourceId);
    }