presto-main/src/main/java/io/prestosql/operator/MultiChannelGroupByHash.java/MultiChannelGroupByHash/updateDictionaryLookBack
class MultiChannelGroupByHash:    private void updateDictionaryLookBack(Block dictionary)
    {
        if (dictionaryLookBack == null || dictionaryLookBack.getDictionary() != dictionary) {
            dictionaryLookBack = new DictionaryLookBack(dictionary);
        }
    }