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