An all-time one to refer back to.

"According to database pioneer Michael Stonebraker, Facebook is operating a huge, complex MySQL implementation equivalent to “a fate worse than death,” and the only way out is “bite the bullet and rewrite everything.”<br />
"Not that it’s necessarily Facebook’s fault, though. Stonebraker says the social network’s predicament is all too common among web startups that start small and grow to epic proportions.<br />
"During an interview this week, Stonebraker explained to me that Facebook has split its MySQL database into 4,000 shards in order to handle the site’s massive data volume, and is running 9,000 instances of memcached in order to keep up with the number of transactions the database must serve. I’m checking with Facebook to verify the accuracy of those numbers, but Facebook’s history with MySQL is no mystery."<br />
You wouldn't really worry that you might become the world's biggest social network when you first typed "create database..."