Forum OpenACS Q&A: Response to Postgres feature questions
Although I know little about it, I suspect the vector-oriented kdb database has similar order-aware functionality, although derived from the APL world, rather than from SQL extensions hacked onto the fundamentally un-ordered relational model. Wippler's Vlerq may be going in a similar direction, although from yet another background.
I've heard that there is some academic work out there on database models which natively understand order, and form more powerful supersets of Codd's relational model. I would have called that a "vector relation model" or an "ordered relation model", but bizarrely, apparently what it's actually called is a "set model"! I have not read it, but The Set Model for Database and Information Systems by Mikhail M. Gilula was recommended to me on that subject.