presto-main/src/main/java/io/prestosql/operator/aggregation/IntervalYearToMonthSumAggregation.java/IntervalYearToMonthSumAggregation/combine
class IntervalYearToMonthSumAggregation:    @CombineFunction
    public static void combine(NullableLongState state, NullableLongState otherState)
    {
        if (state.isNull()) {
            state.setNull(false);
            state.setLong(otherState.getLong());
            return;
        }

        state.setLong(BigintOperators.add(state.getLong(), otherState.getLong()));
    }