hi, which is better between postgresql and mysql in a system with a lot
of queries and where performance is very important ? is possible for
both to have a database cluster? is there anyone who has one ?
Rails tries to stay fairly agnostic about this. I'm personally a *huge* PostgreSQL fan. One major difference is the open source license. PostgreSQL has a BSD license, which is very similar to that of Rails, while MySQL has a dual-license scheme that has some limitations for how you sell your product.
As far as features, PostgreSQL wins because of how far you can go with it... from built-in GIS (PostGIS) to creating stored procedures, triggers, etc... with Ruby... I could go on and on.