Distance lends enchantment to the view—

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 viewVat timeTis the result of evaluating a certain relational ...

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