I recommend you check out CockroachDB. They're approach is to use a Key-Value DB to build a linear-scaling PostgreSQL-like Database. Personally, I'm just wait

I recommend you check out CockroachDB. They're approach is to use a Key-Value DB to build a linear-scaling PostgreSQL-like Database. Personally, I'm just waiting for that to become a bit more stable, but it's probably the most promising in this regard and already very usable.