presto-main/src/main/java/io/prestosql/operator/aggregation/CountIfAggregation.java/CountIfAggregation/combine
class CountIfAggregation:    @CombineFunction
    public static void combine(@AggregationState LongState state, @AggregationState LongState otherState)
    {
        state.setLong(state.getLong() + otherState.getLong());
    }