The distinction between the logical and physical representation of data were recognized in 1978 when ANSI / SPARK committee proposed a generalized framework for database systems. This framework provided a three-level architecture, three levels of abstraction at which the database could be viewed.
Need for Abstraction
The main objective of DBMS is to store and retrieve information efficiently; all the users should be able to access same data. The designers use complex data structure to represent the data, so that data can be efficiently stored and retrieved, but it is not necessary for the users to know physical database
storage details. The developers hide the complexity from users through several levels of abstraction.
Data Independence
Data independence means the internal structure of database should be unaffected by changes to physical aspects of storage. Because of data independence, the Database administrator can change the database storage structures without affecting the users view.
The different levels of data abstraction are:
1. Physical level or internal level
2. Logical level or conceptual level
3. View level or external level
Physical Level
It is concerned with the physical storage of the information. It provides the internal view of the actual physical storage of data. The physical level describes complex low-level data structures in detail.
Logical Level
Logical level describes what data are stored in the database and what relationships exist among those data. Logical level describes the entire database in terms of a small number of simple structures. The implementation of simple structure of the logical level may involve complex physical level structures; the user of the logical level does not need to be aware of this complexity. Database administrator use the logical level of abstraction.
View Level
View level is the highest level of abstraction. It is the view that the individual user of the database has. There can be many view level abstractions of the same data.
Database Instances
Database change over time as information is inserted and deleted. The collection of information stored in the database at a particular moment is called an instance of the database.
Database Schema
The overall design of the database is called the database schema. A schema is a collection of named objects. Schemas provide a logical classification of objects in the database. A schema can contain tables, views, triggers, functions, packages, and other objects.
A schema is also an object in the database. It is explicitly created using the CREATE SCHEMA statement with the current user recorded as the schema owner. It can also be implicitly created when another object is created, provided the user has IMPLICIT SCHEMA authority.
Source: springer-fundamentals-of-relational-database-management-systems-apr-2007
Need for Abstraction
The main objective of DBMS is to store and retrieve information efficiently; all the users should be able to access same data. The designers use complex data structure to represent the data, so that data can be efficiently stored and retrieved, but it is not necessary for the users to know physical database
storage details. The developers hide the complexity from users through several levels of abstraction.
Data independence means the internal structure of database should be unaffected by changes to physical aspects of storage. Because of data independence, the Database administrator can change the database storage structures without affecting the users view.
The different levels of data abstraction are:
1. Physical level or internal level
2. Logical level or conceptual level
3. View level or external level
Physical Level
It is concerned with the physical storage of the information. It provides the internal view of the actual physical storage of data. The physical level describes complex low-level data structures in detail.
Logical Level
Logical level describes what data are stored in the database and what relationships exist among those data. Logical level describes the entire database in terms of a small number of simple structures. The implementation of simple structure of the logical level may involve complex physical level structures; the user of the logical level does not need to be aware of this complexity. Database administrator use the logical level of abstraction.
View Level
View level is the highest level of abstraction. It is the view that the individual user of the database has. There can be many view level abstractions of the same data.
Database Instances
Database change over time as information is inserted and deleted. The collection of information stored in the database at a particular moment is called an instance of the database.
Database Schema
The overall design of the database is called the database schema. A schema is a collection of named objects. Schemas provide a logical classification of objects in the database. A schema can contain tables, views, triggers, functions, packages, and other objects.
A schema is also an object in the database. It is explicitly created using the CREATE SCHEMA statement with the current user recorded as the schema owner. It can also be implicitly created when another object is created, provided the user has IMPLICIT SCHEMA authority.
0 comments :
Post a Comment
Note: only a member of this blog may post a comment.