presto-main/src/main/java/io/prestosql/operator/MultiChannelGroupByHash.java/GetNonDictionaryGroupIdsWork/getResult
class GetNonDictionaryGroupIdsWork:        @Override
        public GroupByIdBlock getResult()
        {
            checkState(lastPosition == page.getPositionCount(), "process has not yet finished");
            checkState(!finished, "result has produced");
            finished = true;
            return new GroupByIdBlock(nextGroupId, blockBuilder.build());
        }