@Override
    public void reset () {
      isNull = true;
      usingRegularDecimal = false;
      sum = 0;
      regularDecimalSum.setFromLong(0);
    }
Value is explicitly (re)initialized in reset()
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/aggregates/VectorUDAFSumDecimal64ToDecimal.java