presto-main/src/main/java/io/prestosql/operator/MultiChannelGroupByHash.java/GetDictionaryGroupIdsWork/GetDictionaryGroupIdsWork
class GetDictionaryGroupIdsWork:        public GetDictionaryGroupIdsWork(Page page)
        {
            this.page = requireNonNull(page, "page is null");
            verify(canProcessDictionary(page), "invalid call to processDictionary");

            this.dictionaryBlock = (DictionaryBlock) page.getBlock(channels[0]);
            updateDictionaryLookBack(dictionaryBlock.getDictionary());
            this.dictionaryPage = createPageWithExtractedDictionary(page);

            // we know the exact size required for the block
            this.blockBuilder = BIGINT.createFixedSizeBlockBuilder(page.getPositionCount());
        }