presto-main/src/main/java/io/prestosql/operator/MultiChannelGroupByHash.java/MultiChannelGroupByHash/AddDictionaryPageWork
class MultiChannelGroupByHash:        public AddDictionaryPageWork(Page page)
        {
            verify(canProcessDictionary(page), "invalid call to addDictionaryPage");
            this.page = requireNonNull(page, "page is null");
            this.dictionaryBlock = (DictionaryBlock) page.getBlock(channels[0]);
            updateDictionaryLookBack(dictionaryBlock.getDictionary());
            this.dictionaryPage = createPageWithExtractedDictionary(page);
        }