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CHAPTER 27 A COMPARISON BETWEEN ODMG AND THE THIRD MANIFESTO
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While we are on the subject of inheritance, incidentally, the ODMG book includes a nice example of how difficult it can be to get the type hierarchy right: For example, ASSOCIATE_ PROFESSOR is a subtype of PROFESSOR ... Where an object of type PROFESSOR can be used, an object of type ASSOCIATE_PROFESSOR can be used instead, because ASSOCIATE_ PROFESSOR inherits from PROFESSOR. But surely professors have properties perhaps tenure that associate professors do not In other words, is not the hierarchy (at best) upside down ODMG does not appear to distinguish between value and variable substitutability.
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Object Definition Language
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The foregoing subsections summarize the major aspects of the Object Model (though we have skipped over features that we regard as secondary, such as details of the catalog or metadata and details of recovery and concurrency control). Now we turn to the Object Definition Language, ODL. ODL is basically a language that provides a concrete syntax for the specification of object types that conform to the ODMG Object Model. Note: ODL supports the definition of operator specification signatures (Manifesto book terminology) including the names of any exceptions that might be raised by the operation in question but does not provide a means of writing the code to implement such operations. Presumably that code must be written in a language such as C++ or Java. The chapter of the ODMG book that discusses ODL gives a number of examples, together with a complete definition of ODL syntax (a BNF grammar), but says almost nothing about semantics. Possibly the reader is supposed to have read the OMG specifications (on which ODL is based) first. In any case, we omit the details of ODL here since (as we say in the Manifesto book) we do not regard matters of mere syntax as being very important.
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Object Interchange Format
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To quote the ODMG book, the Object Interchange Format (OIF) is a specification language used to dump and load the current state of an object database to or from a file or set of files. As such, it is not very germane to the present discussion, and we therefore skip the details.
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Object Query Language
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The Object Query Language OQL might be characterized as a large superset of a small subset of SQL, with incompatibilities. It is not the ODMG Object Manipulation Language (as noted in the introduction to this chapter, no such language exists); rather, it is, specifically, a query language that supports nonprocedural retrieval (only)11 of data stored in an ODMG database. As such, it supports a variety of operators that are not part of the Object Model per se. It is not computationally complete. We choose not to provide anything close to a complete description of OQL here. Suffice it to say that it supports: SQL-style SELECT-FROM-WHERE queries against sets, bags, lists, and arrays Analogs of the SQL GROUP BY, HAVING, and ORDER BY constructs
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11. What happens if an OQL query invokes an update operator does not seem to be defined.
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CHAPTER 27 A COMPARISON BETWEEN ODMG AND THE THIRD MANIFESTO
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Union, intersections, and differences, and special operations for lists and arrays (e.g., get the first element ) Path expressions for traversing relationships There appear to be quite a few detail-level incompatibilities, of both style and substance, between OQL and the Object Model. For example, [certain] expressions yield objects without identity (but surely objects always have identity in the Object Model ); OQL allows us to call a method (but methods are defined earlier in the book to be part of the implementation, not part of the model); [we can retrieve] the ith element of an indexed collection (the term indexed here does not mean what it means earlier in the book); and so on. It should be noted too that the semantics of nulls and certain query constructs are presumably unintentionally different from those of their SQL counterparts.
Summary
The basic idea behind ODMG, in a nutshell, is (of course) to allow many different data structures sets, bags, lists, arrays, and so on to be used for data in the database and to expose them all to the user ( persistence orthogonal to type ). We reject this idea for reasons explained in detail in reference [4] and sketched in the annotation to that reference in the final section of this chapter. We now proceed to consider the question of how ODMG measures up to the various prescriptions, proscriptions, and suggestions defined formally and explained in (the third edition of) the Manifesto book. What follows thus consists essentially of a series of point-bypoint comparisons of pertinent ODMG features with those prescriptions, proscriptions, and suggestions. The comparisons are presented mostly as bald statements of fact; we (mostly) do not restate opinions, give value judgments, or comment on the relative severity of various points. Also, we use the terms conforms and fails, in boldface, to indicate our general finding in connection with each point. Very often these terms have to be qualified, and sometimes both are used in connection with the same point. For example, ODMG sometimes conforms to (say) a certain prescription in some respects but fails in others; sometimes it conforms as far as it goes but does not go far enough; sometimes it fails not because of specifics of the feature at hand, but rather because that feature depends on some other feature on which it fails in turn. Such dependencies are appropriately indicated.
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