Most importantly is SQLite3 as the new database we’ll configure for by default when you run the rails generation command without any specification. This change comes as SQLite3 is simply an easier out of the box experience than MySQL. There’s no fussing with GRANTs and creates, the database is just there. This is especially so under OS X 10.5 Leopard, which ships with SQLite3 and the driver gems preinstalled as part of the development kit.
If you want to preconfigure your database for MySQL (or any of the other adapters), you simply do “rails -d mysql myapp” and everything is the same as before. But if you’re just playing with a new application or building a smallish internal tool, then I strongly recommend having a look at SQLite3. Thanks to the agnostic db/schema.rb, it’s as easy as changing your config/database.yml to switch from SQLite3 to MySQL (or another database) as soon as your load warrants it. [OK. Nice to the tweaks flowing…]
Source: Riding Rails