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CHAPTER 5 ON THE LOGICAL DIFFERENCE BETWEEN MODEL AND IMPLEMENTATION
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I d also like to comment on some of the more detailed points raised in the foregoing quotes. In answer to the question as to whether arrays, lists, and the like complicate the query language, Mary Loomis responds that OQL is fully specified in about 17 pages2 and takes a side swipe at the SQL standard. There are several points I d like to make in this connection: First, I m no fan of SQL, but it s only fair to point out that the SQL standard addresses much more of the total database problem than OQL does, and that s one of the reasons why it s a very thick document (much thicker now than it was in 1994, incidentally). On the other hand, I suspect that Mary Loomis would claim and if so I would agree with her that if we were to cut the SQL standard down to just the portions that address the kinds of issues that OQL does, then it would still be much thicker than the OQL specification. As I ve had occasion to note elsewhere, SQL is a poorly designed language by just about any standard (pun intended); thus, I m quite prepared to accept the notion that OQL, if we limit our attention to the relational portions of that language, might be a better language than SQL (perhaps less redundant, and certainly more orthogonal). Even conceding the previous point, however, the fact remains that, pace Mary Loomis, OQL is more complicated than it needs to be: It supports bags, lists, and arrays, and so it clearly needs a set of bag operators, a set of list operators, and a set of array operators. In fact, it is axiomatic that if we have N different ways to represent data, then we need N different sets of operators. And if N > 1, then we have more operators to implement, document, teach, learn, remember, and use. But those extra operators add complexity, not power! There s nothing useful that can be done if N > 1 that can t be done if N = 1. Onward. The next quote includes the claim that relationships and sequence are semantic constructs and the further claim that the fact that CODASYL included such constructs doesn t mean that ODMG represents a step backward (both quotes slightly reworded here). Well, it seems to me that we have here more of the same confusions once again. Consider relationships, for example. Of course relationships are a semantic construct but that s not the issue! The issue, rather, is how that semantic construct should be represented in the model (indeed, the distinction between a concept per se and how that concept is represented is yet another of the great logical differences). The relational model gives us a uniform way to represent all entities and all relationships,3 and the advantages of that uniformity of representation are clear and well understood (at least, they should be). And then, if pointers turn out to be a good way to implement certain relationships, well, fine, go ahead and use them but in the implementation, not in the model. The trouble with CODASYL and ODMG and other suchlike schemes is that they use pointers in the model (thereby making them user-visible and leading to a pointerchasing, manual-navigation style of access). In my opinion, therefore (pace Mary Loomis again), ODMG s inclusion of such pointers definitely is a step backward. The arguments of the previous paragraph also serve in part to counter the claims in the third quote (repeated for convenience):
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2. Those 17 pages became 44 in the most recent version of ODMG. What s more, my copy is scribbled all over with questions that I couldn t find the answer to, so I m somewhat skeptical as to whether those 44 pages really do fully specify the language as claimed. 3. It s relevant to remind you that relationships are entities, anyway.
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