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

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