Everyone should try PostgreSQL with hstore (and JSONB now, too!).

This is a key/value store inside an RDBMS that just works, and it works great!

I converted a crappy sloppy super messy 1000+ column main table in a ~800GB database to use hstore, it was, in real world benchmarks, between 7x and 10,000x (yes, really, ten thousand times) faster.

The CEO of the company who had a technical say in everything, and was very proud of his schema "wasn't excited" and it never happened in any production instance.

I've left since then, and the company has made very little advancement, especially when it comes to their database.

Really, just use hstore. Try it out. The syntax is goofy, but... I mean, SQL itself is a little bit goofy, right?