The default assumption in web development is that serious applications run on serious databases, and serious databases means a separate server process, connection pooling, user management, and a configuration file that will eventually be wrong in a way that takes an afternoon to diagnose. MySQL and PostgreSQL are excellent databases. They are also, for the median small site, a solution in search of a problem — infrastructure designed for concurrency, scale, and replication requirements that don’t exist at any traffic level the site will realistically see for years.