Well, support cross sharding `join`, `subslect` and `group by` is not easy, I see Vitess does not fully support them now. I am the only guy to develop Mixer ju

Well, support cross sharding `join`, `subslect` and `group by` is not easy, I see Vitess does not fully support them now. I am the only guy to develop Mixer just for fun with little time, and have to improve it step by step. Any help is welcome. Btw, I have not used MongoDB, I will try to see it later.