None of the stores I mentioned use mysql. I'm not talking about scaling mysql, I'm talking about scaling relational databases, specifically the comment about jo

None of the stores I mentioned use mysql. I'm not talking about scaling mysql, I'm talking about scaling relational databases, specifically the comment about joins. Joins arent a big deal, just join on (at least) the shard key, or for smaller hash joins ship the small side of the join. If you have multiple kinds of joins keep multiple copies with different shard keys. Yes, (monoidal) aggregations are possible with this, yes subqueries are possible with this.