The Death of the Relational Database argues that relational databases aren’t good for applications that may add various additional relationships with new features, because the tables store not just the “objects” but the relationships between them.
In the early days at Mosuki, Jonathan implemented a table to store “certifications,” which are essentially directed, labeled edges connecting arbitrary rows, or objects. These edges can connect any two rows in any of several different database tables. And this power lets us implement major features that we didn’t plan for originally, like adding iCalendar feeds, discussion forums, scenes, places, and a myriad of object-specific user preferences to the site, with a minimum of schema restructuring. Often we can add whole features without touching the schema.
This table is also the largest, and most problematic speed-wise as it requires two joins instead of one (although Ross has got it pretty much under control now), which I think speaks to the original article’s point that relational databases aren’t the best choice for more free-form applications.