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 exampleObject 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
OID must be provided so that messages can
be sent to those instances even after the program
that issued the query has terminated.
A View Mechanism for Object-oriented databases
By Žilvinas Kučinskas
