public static IAggregableReduceOp<Writable, List<Writable>> reduceLongColumn(List<ReduceOp> lop,
                    boolean ignoreInvalid, ColumnMetaData metaData) {

        List<IAggregableReduceOp<Long, Writable>> res = new ArrayList<>(lop.size());
        for (int i = 0; i < lop.size(); i++) {