And what about noSQL databases? Everyone says that they are faster and more scalable than SQL. Moreover MongoDB have interface in JSON, so it fits with default bitcoind interface.
BAD idea. NoSQL is only faster because it doesn't do all that ACID checking that transaction-safe RDBMS does. While it's perfectly fine for data that doesn't need to be consistent i.e. nobody's going to particularly care if half the world saw your twit #156483 and the other half only get updated to #156482 for the next 2 minutes. But for financial transaction, having inconsistent data and no referential integrity is malpractice and asking outright to be scammed.