presto-main/src/main/java/io/prestosql/operator/aggregation/ChecksumAggregationFunction.java/ChecksumAggregationFunction/combine
class ChecksumAggregationFunction:    public static void combine(NullableLongState state, NullableLongState otherState)
    {
        state.setNull(state.isNull() && otherState.isNull());
        state.setLong(state.getLong() + otherState.getLong());
    }