Additional comparison recommendations
To close out this discussion, here are a few additional rules and recommendations for comparison methods:
· MAP and ORDER cannot coexist in the same object type; use one or the other.
· Oracle recommends MAP when you have a large number of objects to sort or compare, as in a SQL statement. This is because of an internal optimization that reduces the number of function calls. With ORDER, the function must run once for every comparison.
· Oracle ignores the method names; you can call them anything you want.
· Subtypes can include MAP methods, but only if the supertype also has one.
· Subtypes cannot have ORDER methods; you'll have to put all the comparison "smarts" into the supertype.
21.3 Object Views
Although Oracle's object extensions offer PL/SQL programmers rich possibilities for the design of new systems, it's unlikely that you will want to completely reengineer your existing systems to use objects. In part to allow established applications to take advantage of the new object features over time, Oracle provides object views. This feature offers several unique advantages:
"Object-ification" of remote data
It's not yet possible in Oracle9i to use the object tables and physical REFs across a distributed database, but you can create object views and virtual REFs that cast remote relational data as objects.
Virtual denormalization
In a relational database or even an object-relational database, you will usually find relationships modeled in only one direction. For example, a book has some number of subjects. With an object view, it's easy to associate a column that provides the inverse mapping; for example, a subject object could include a collection of REFs that point to all of the books in that subject.
Efficiency of object access
In Oracle Call Interface (OCI) applications, object programming constructs provide for the convenient retrieval, caching, and updating of object data. By reducing trips between application and database server, these programming facilities may provide performance improvements, with the added benefit that application code can be more succinct.
Greater flexibility to change the object model
Although Oracle9i has tremendous abilities in the area of type evolution, adding and removing object attributes still cause table bits to move around on the disk, which administrators may be loath to do. Recompiling object views suffers no such consequences.
On the other hand, there are some disadvantages to using object views:
View performance
Object views are still views, and some Oracle shops are generally leery of the performance of any view.
No virtual REFs
You cannot store virtual REFs in the database; instead, they get constructed on the fly. This may present some challenges if you someday want to convert those object views into object tables.
Other features of Oracle can improve the expressiveness of any types of views, not just object views. Two such features that are not strictly limited to object views are collections and INSTEAD OF triggers:
Collections
Consider two relational tables with a simple master-detail relationship. You can create a view portraying the detail records as a single nonscalar attribute (collection) of the master.
INSTEAD OF triggers
In addition, by using INSTEAD OF triggers, you can tell Oracle exactly how to perform inserts, updates, and deletes on the view.
From an object perspective, there is one slight disadvantage of object views when compared to comprehensive reengineering: object views cannot retrofit any benefits of encapsulation. Insofar as any applications apply INSERT, UPDATE, and DELETE statements directly to the underlying relational data, they may subvert the benefits of encapsulation normally provided by an object approach. Object-oriented designs typically prevent free-form access directly to data. However, because Oracle supports neither private attributes nor private methods, the incremental sacrifice here is small.
If you do choose to layer object views on top of an existing system, it may be possible for new applications to enjoy incremental benefit, and your legacy systems are no worse off than they were before. Figure 21-2 illustrates this use of object views.