Data Warehousing Inheritance Oracle Tips by Burleson Consulting

High Performance Data Warehousing

Inheritance

Not all classes within a generalization hierarchy will have objects associated with them. The object-oriented paradigm allows for abstraction, which means that a class may exist only for the purpose of passing inherited data and behaviors. The classes vehicle and car would probably not have any concrete objects, while objects within the van class would inherit from the abstract vehicle and car classes. Multiple inheritance is also demonstrated by the amphibian_car class. Any instances of this class will inherit data and behaviors from both the car and the boat classes.

It is important to note the tremendous difference between one-to-many relationships and IS-A relationships. In the previous example, this entire class hierarchy describes vehicles which are associated with the item entity in the overall database. Class hierarchies do not imply any data relationships between the classes. While one customer may place many orders, it is not true that one car may have many sedans.

Polymorphism

Polymorphism is the ability of different objects to receive the same message and behave in different ways. This concept has many parallels in the real world. An event such as a volcanic eruption may have many different effects on living things in the area. The poisonous gasses may kill all air-breathing animals while at the same time nourish small marine organisms. In this scenario, the single behavior of eruption has different effects on objects within the animal class. Another analogy can be found in the business world. For a personnel manager, the event of promotion will cause different behaviors depending on the class of employee which receives the promotion. Management objects will receive stock options and country club memberships which are not offered to staff objects.

Ronald Popeil was a master of polymorphism. Many folks remember the heyday of Ronco and Popeil, where polymorphic products were advertised at the national level. Consider the statement: It's a hair cream AND a floor wax. If this is indeed true, the method spread_it_on would invoke very different processes depending upon whether we are applying the cream to a floor or a person's head.

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This is an excerpt from "High Performance Data Warehousing", copyright 1997.

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