Awesome, Thanks for the reply. My use cause is barely out of the "toy app" range. We only do a handful of events each year and they only draw around 100 attendants. We're talking a very small amount of data. When you do the PostgreSQL and Riak combo is it ever on the same/related dataset. What are you using at the application layer? I'm building this in rails and I feel like it would be better to store the structured fields in MySQL and the variant data in something NoSQL. But I haven't read much into using active record with two different persistence layers. That's interesting about PostgreSQL with indexable BJSON. I've only ever used MySQL, I really need to check out Postgres.
It's easier to go from something more highly structured to something looser. Start with a relational db and let it grow then pay attention to what data gives you the most scale pain and try to move that out to Riak / Cassandra / etc...
Don't prematurely scale, just pay attention to your metrics, scale vertically first, then tackle the very specific pain points.
