presto-main/src/main/java/io/prestosql/operator/MultiChannelGroupByHash.java/GetRunLengthEncodedGroupIdsWork/getResult
class GetRunLengthEncodedGroupIdsWork:        @Override
        public GroupByIdBlock getResult()
        {
            checkState(processFinished);
            checkState(!resultProduced);
            resultProduced = true;

            return new GroupByIdBlock(
                    nextGroupId,
                    new RunLengthEncodedBlock(
                            BIGINT.createFixedSizeBlockBuilder(1).writeLong(groupId).build(),
                            page.getPositionCount()));
        }