@Override
    protected void onDeserialize(Deserializer buf) {
        super.onDeserialize(buf);
        count = buf.getLong(null);
        sum = (NumericResultNode)deserializeOptional(buf);
    }
Returns the average value of the results. Because the result can be any numeric type, this method returns a
{@link NumericResultNode} object.

@return The average result value.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/AverageAggregationResult.java