presto-main/src/main/java/io/prestosql/operator/aggregation/DecimalSumAggregation.java/DecimalSumAggregation/combine
class DecimalSumAggregation:    public static void combine(LongDecimalWithOverflowState state, LongDecimalWithOverflowState otherState)
    {
        state.setOverflow(state.getOverflow() + otherState.getOverflow());

        if (state.getLongDecimal() == null) {
            state.setLongDecimal(otherState.getLongDecimal());
        }
        else {
            accumulateValueInState(otherState.getLongDecimal(), state);
        }
    }