presto-main/src/main/java/io/prestosql/operator/aggregation/RealSumAggregation.java/RealSumAggregation/combine
class RealSumAggregation:    @CombineFunction
    public static void combine(@AggregationState NullableDoubleState state, @AggregationState NullableDoubleState otherState)
    {
        if (state.isNull()) {
            if (otherState.isNull()) {
                return;
            }
            state.setNull(false);
            state.setDouble(otherState.getDouble());
            return;
        }

        if (!otherState.isNull()) {
            state.setDouble(state.getDouble() + otherState.getDouble());
        }
    }