@Override
    protected void onDeserialize(Deserializer buf) {
        super.onDeserialize(buf);
        sum = (SingleResultNode)deserializeOptional(buf);
    }
Sets the sum of all results in this.

@param sum The sum to set.
@return This, to allow chaining.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/SumAggregationResult.java