public void sumValue(HiveDecimalWritable writable) {
      if (isNull) {
        // Make a copy since we intend to mutate sum.
        sum.set(writable);
        isNull = false;
      } else {
        sum.mutateAdd(writable);
      }
    }
class for storing the current aggregate value.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/aggregates/VectorUDAFSumDecimal.java