Updates in object-oriented database systems are studied. It is pointed out that, owing to the richness of the object-oriented data model, updating an object may affect the object's position in the class lattice. Three types of update operations on objects are considered: adding, dropping, and modifying instance variables. An update may cause object migration, that is, the updated object may change its current class. As the user may not be aware of the side effects of the update, the system has to take some actions to complete the user's update. Classes in a database schema are classified into three types: static, partially dynamic, and dynamic. This classification is based on the way in which an object in a class may migrate. It is shown that the notion of object migration affects answering queries in temporal object-oriented databases. The history of an object will be distributed among several classes in the scheme. Two approaches to determine the class of an object's version are presented.
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