for your reading pleasure, please take a look at this paper: http://www.hpl.hp.com/techreports/tandem/TR-85.6.pdf

what they describe there basically is the current state of affairs: different database technology for different purposes (in-memory, hierarchical and journal). I love the paper for it shows how things do actually not change.

(re that Tandem paper: it's IMS / a hierarchical DB that they describe though)

I'm not sure what you're referring to; Jim Gray's work looks pretty relational to me. http://research.microsoft.com/en-us/um/people/gray/papers/Be... is just one example.