Dusting off another post stashed in my Bloglines aggregation, Mark Fletcher discussed why Bloglines doesn't use a relational DB underneath. Sleepycat, longtime purveyors of fine Berkeley DB style, associative, key indexed database software, is the engine that manages Bloglines data. Similar to the way you can punt an RDBMS if you don't really need all of its persistence and transaction management, if you're not doing relational queries, you can drop kick the model, and attendant overhead as well.