[beagle3]

The original B-Trees as described by Bayer were "intrusive" (like implemented here), in the sense that every node contains keys, values, and pointers.

There are variants such as the B+Tree http://en.wikipedia.org/wiki/B%2B_tree , which stores only keys in nodes and chains blocks - which is more efficient in range scans and less in general retrieval; And the http://en.wikipedia.org/wiki/B*-tree which is more densely packed.