From his article: <i>Many common types of data are intrinsically heirarchical and loosely structured, so storing them in an RDBMS can be difficult. Examples: books, articles, and most other written documentation. HTML. Search indexes. Filesystems. Game worlds. Classical music scores. It's just easier to model some things this way.</i><p>I don't see how to interpret this as anything other than a document database. Could you explain to me?