public void sumValue(long value) {
      if (isOverflowed) {
        return;
      }
      if (isNull) {
        sum = value;
        isNull = false;
      } else {
        sum += value;
        if (Math.abs(sum) > outputDecimal64AbsMax) {
          isOverflowed = true;
        }
      }
    }
Value is explicitly (re)initialized in reset()
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/aggregates/VectorUDAFSumDecimal64.java