Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!cs.utexas.edu!uunet!daitc!daitc.daitc.mil From: jkrueger@daitc.daitc.mil (Jonathan Krueger) Newsgroups: comp.databases Subject: Re: Relational Model (Was Re: Oracle: Previous Record) Keywords: relational empirical support Message-ID: <606@daitc.daitc.mil> Date: 22 Jul 89 18:41:06 GMT References: <18886@sequent.UUCP> Sender: jkrueger@daitc.daitc.mil Reply-To: jkrueger@daitc.daitc.mil (Jonathan Krueger) Organization: DTIC Special Projects Office (DTIC-SPO), Alexandria VA Lines: 59 In-reply-to: normb@sequent.UUCP (Norm Browne) In article <18886@sequent.UUCP>, normb@sequent (Norm Browne) writes: >I could find no substantiation >that the relational model _really_ represented real world applications. Lewis Carroll used to enjoy describing his map of England, which, he said, was better than all others. It represented the territory more accurately. In fact, it showed every detail: private roads, property boundaries, interiors of buildings, items on shelves, every leaf on every tree, the contents of every cell in every leaf. It managed this feat by using a 1:1 scale, which also lent the advantage that you could explore it quite literally. Everything you could see in the map was exactly true of England. In fact, it was England. England is a map of England. A perfect one. Also, a useless one: the first thing you need is a map of this map, of the sort you can carry around with you, for instance. Carroll's point was that all real maps are distortions, by necessity. The reasoning is by the contrapositive from the premise: if a map is NOT a distortion, it's not useful. This premise is easily granted by examining the denerate case. If the map is indistinguishable from the territory, in no way does it distort, and it's also completely useless (as a map). Therefore, if a map would be useful, it MUST differ in some way from the original. (Of course, the mere fact of differences does NOT prove usefulness: differences are necessary but not sufficient to make the map useful.) The same point can be made informally: why do we want maps anyway? Very much because they summarize and abstract some facts about the territory. In so doing, they all distort. Roads are shown as wider than they really are, for instance. Were they shown to scale, they'd be hard to draw and even harder to read. Map features such as borders are drawn inaccurately or out of scale, or omitted altogether. Were they correct in every particular, some would obscure the map features that users need to find. Even those that didn't would still drive up the production time and cost to years and millions. Users might prefer maps showing 90% as many details relevant to their use which cost 10% as much and were available in their lifetimes. So, have I made my point here? Or do I have to draw a map for ya? :-) Maps are models. Models are the general case. Good models MUST distort. Therefore, the question is not how well the model represents the real world, but how usefully. That the model omits or simplifies its account of the real world is NOT a fault, but rather a prerequisite for it ever to be useful. To avoid a potential area of confusion, the word `application', as in "the relational model _really_ represented real world applications", has two common usages: the task to be performed, and a computer program or set of them that is said to automate the task. In this article I have meant the first usage. The second may have been intended by Norm, however. If so, he's saying that he has no evidence that two models represent each other. This is probably true, and of no great consequence. How programs resemble each other is about as interesting as how maps resemble each other. How maps represent the territory in useful ways is more interesting, and similarly how programs automate tasks. -- Jon --