View Updating and Relational Theory

Chapter 3. The View Concept: A Closer Look

Distance lends enchantment to the view

— Thomas Campbell:

Every scientific discipline has its share of unsolved problems. In mathematics, for example, there’s the Riemann Hypothesis, which nobody has managed to prove or disprove in over 150 years; in computer science, there’s the “P = NP?” question, which is still open after some 40 years; in physics and cosmology, there’s the long search for a “theory of everything,” which remains—in many people’s opinion, though possibly not in everyone’s—just that, a search; and in database theory, there’s the problem of view updating. Now, I obviously don’t mean to suggest that the problem of view updating is in the same league as the Riemann Hypothesis or the “P = NP?” question or some hypothetical “theory of everything”;^[35] however, I do claim it’s of considerable practical importance and much theoretical interest. In this chapter, therefore, I want to lay some of the groundwork that’s necessary for a systematic attack on that problem.

I begin with the trite observation that a view is a relvar—a virtual relvar, to be precise, or in other words a relvar that “looks and feels” just like a real, or base, relvar but (unlike a real or base relvar) doesn’t exist independently of other relvars; rather, it’s defined in terms of, or derived from, such other relvars. Here’s a definition:

Definition: A view is a derived, virtual relvar. The value of view V at time T is the result of evaluating a certain relational ...