presto-main/src/main/java/io/prestosql/operator/aggregation/LongSumAggregation.java/LongSumAggregation/combine
class LongSumAggregation:    @CombineFunction
    public static void combine(@AggregationState LongLongState state, @AggregationState LongLongState otherState)
    {
        state.setFirst(state.getFirst() + otherState.getFirst());
        state.setSecond(BigintOperators.add(state.getSecond(), otherState.getSecond()));
    }