The co-founder of Reddit gave a presentation on issues they had while scaling to millions of users. A summary is available here.
What surprised me is point 3:
Instead, they keep a Thing Table and a Data Table. Everything in Reddit is a Thing: users, links, comments, subreddits, awards, etc. Things keep common attribute like up/down votes, a type, and creation date. The Data table has three columns: thing id, key, value. There’s a row for every attribute. There’s a row for title, url, author, spam votes, etc. When they add new features they didn’t have to worry about the database anymore. They didn’t have to add new tables for new things or worry about upgrades.
This seems like a terrible idea to me, but it seems to have worked out for Reddit. Is it a good idea in general, though? Or is it a peculiarity of Reddit that happened to work out for them?