A View Mechanism for Object-oriented databases

According to Elisa Bertino paper

Content

Introduction
View Definition and Semantics
Views for Schema Evolution
Conclusion

Relational data model

A view is defined as a virtual relation derived

by a query on one or more stored relations

USAGE

Data protection

User convenience

Important question

Is the concept of views in object-oriented databases

is still useful?

How about?

Graduate

Student ->

Undergraduate

Object oriented data models

Objects (identifier - OID) - state, values, methods

Classes

Inheritance

Database schema example

Object oriented predicate calculus

Target - what must be retrieved

Range - classes to which the query applies

Qualification - boolean combination of simple predicates

Example

Retrieve the last names of all permanent employees and consultants living in Rome, working in the technical staff of a team sponsored by a company located in Turin, and such that if they are permanents their salary is higher than 4000, if they are consultants their daily wage is higher than 500.

Query

Organization of an Object-oriented schema with views

View derivation - a view derivation relationship

between a class C and a view V denotes that

V is defined "on top of" C, that is, C is a base class of V

Important point

There is no relationship between class and the view

in this case

View definition language

Methods

System should determine the properties
used in each method implementation

Example

Views for schema evolution

Now it is possible for a user to define his own

modified schema without affecting other users.

Basic changes

Class content - adding, dropping, renaming properties

Inheritance hierarchy - changing methods,

inheriting properties via additional properties

Example - dropping property

Other example

Suppose that if a machine is produced in the USA, the price is 20% less there than the price in Italy, while in other countries 10% less.

Merge of classes in an inheritance hierarchy

!!! This may not be possible if object migration among classes is not supported

Single view? Hierarchy of views?

Mapping methods

Toy implementation of method wage

self is used to denote the object to which the message

invoking the method has been sent

Modification to aggregation hierarchies

Telescope method.. <- easy one

connection

connection by base-instance

Conclusion

MAJOR issue in views for object-oriented databases is that
OID must be provided so that messages can
be sent to those instances even after the program
that issued the query has terminated.