[fendale]

The MySQL clustered index (and Oracle IOT) are, in my opinion, often over looked powerful features. To pick an example I recently encountered ... I have an Rails app to implement a RSS reader. Each user has many feeds that get added over time. If you cluster the feeds on user_id then all the feeds for a given user will be in a single database block or two, so reading them out is super efficient. If they are in a standard heap table, (or the way Rails creates the table by default, clustered on ID), then reading the list of N feeds could require N database block reads in the extreme case. Same goes for all articles on a given feed - cluster the related articles together and its very efficient to find the most recent articles for a feed.