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CHAPTER 5 ON THE LOGICAL DIFFERENCE BETWEEN MODEL AND IMPLEMENTATION
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An object DBMS builds these relationships into the object database and can use them directly at run time ... By contrast, a relational DBMS must recreate relationships at run time using joins, based upon instructions from an application. Note the tacit admission here that the purpose of including relationships meaning user-visible pointers in the model is indeed performance ... To paraphrase: The system can follow the pointers directly at run time instead of having to do joins. Again, therefore, it seems to me that object advocates complicate their model precisely because they don t understand why the model and the implementation were supposed to be kept rigidly separate in the first place. I note in passing that the SQL standard also includes such user-visible pointers, thanks to the mechanism it calls REF types further proof, if such were needed, that the people responsible for the design of SQL either don t understand the relational model or don t agree with it. REF types constitute a huge departure from the relational model, and I ve explained elsewhere some of the problems they cause; see my papers Don t Mix Pointers and Relations! and Don t Mix Pointers and Relations Please! (both in Relational Database Writings 1994 1997, Addison-Wesley, 1998). I ve never seen a good justification for the introduction of REF types into SQL (and believe me, it s not for lack of asking); in fact, I think its support for REF types means SQL finally has to abandon any claim it might ever have had to being relational. Analogous remarks apply to the semantic construct sequence. It s true that sequence is a semantic construct, but again that s not the issue; rather, the issue is how to represent that construct at the model level. Introducing an explicit sequence construct into the model is a bad idea for exactly the same kinds of reasons that introducing an explicit relationship construct into the model is a bad idea. The fourth quote betrays some related confusions. To repeat: To run through the rows [of a table] in ... sequence requires the relational DBMS to sort the rows. Well, of course the desired logical sequence might be physically represented by an index, or by a pointer chain, or even by physical contiguity (meaning in every case that there s certainly no need for the relational DBMS to sort the rows at least, not at run time). All of these possibilities are supported in SQL systems today! In fact, the options available for physically implementing some logical sequence are exactly the same in a relational system as they are in an object system. The difference, of course, is that object systems typically require the user to choose the implementation (by choosing to use, say, a list instead of an array), while relational systems keep the choice where it belongs, in the implementation. I ll close this section with a Ted Codd quote. This exchange occurred on a conference panel, as I recall. We were discussing this very issue the issue, that is, of whether the model should include a variety of different data structures and Ted said this: If you tell me you have 50 different ways of representing data in your system at the logical level, then I ll tell you that you have 49 too many. Quite right.
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CHAPTER 5 ON THE LOGICAL DIFFERENCE BETWEEN MODEL AND IMPLEMENTATION
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Examples of Confusion: Objects
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The quotes in the previous section all come from the object world. So do those in the present section, but they re more of a potpourri: R. G. G. Cattell, Object Data Management (revised edition), Addison-Wesley, 1994: [Only] an object s methods may examine or update [the private memory of the object]; the methods are public. I find this quote confused for the following reason: To say that methods are public means they re part of the model; to say they can examine or update private memory means they re part of the implementation. Quotes like this one lend weight to my contention that object advocates typically fail to make a clear distinction between model and implementation. Malcolm Atkinson et al., The Object-Oriented Database System Manifesto, Proc. First International Conference on Deductive and Object-Oriented Databases, Kyoto, Japan, 1989: We are taking a Darwinian approach: We hope that, out of the set of experimental prototypes being built, a fit model will emerge. Perhaps the writers here might legitimately claim not to be confused over model vs. implementation, but they certainly don t seem to agree that the model should be defined first and implementations should be built second! Instead, they seem to be saying that we should write a bunch of code first and see what happens ... In this connection, it s relevant to mention that that s exactly what happened with the old-fashioned hierarchic and network models : The implementations were built first and the corresponding models were then defined, after the fact, by a process of induction from those implementations in other words, by guesswork. And it would be a pity to repeat such a huge logical mistake; yet that seems to be exactly what some people want to do. Myself, I think we should have some idea as to what we re trying to do before we actually do it. And here are two more quotes from that same interview with Mary Loomis (Data Base Newsletter 22, No. 6, November/December 1994): Even with immutable objects, however, it still is important to separate the issue of object identity from how that identity is implemented. The pattern that represents a fingerprint is an immutable object, yet the designer probably would not want to use that fingerprint s bit pattern as the OID. Interviewer: Relational purists claim that object identifiers are just pointers in disguise, and that all pointers and navigational access using them should be avoided. Do you agree Response: If object identifiers were truly pointers, I would agree. Object identifiers, however, are logical references, which eventually lead to an address or pointer. Object identifiers used as references represent logical relationships between objects. Because they are logical, [they] must be able to cross database and processor boundaries. Ordinary pointers won t work in that kind of broad environment.
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