public boolean merge(UDAFAvgState o) {
      if (o != null) {
        state.mSum += o.mSum;
        state.mCount += o.mCount;
      }
      return true;
    }
Merge with a partial aggregation.

This function should always have a single argument which has the same
type as the return value of terminatePartial().
contrib/src/java/org/apache/hadoop/hive/contrib/udaf/example/UDAFExampleAvg.java