Data Abstraction in DBMS
Database systems are made-up of complex data structures. To ease the user interaction with database, the developers hide internal irrelevant details from users. This process of hiding irrelevant details from user is called data abstraction.
We have three levels of abstraction:
Physical level or Intrenal level :
This is the lowest level of data abstraction. It describes how data is actually stored in database. You can get the complex data structure details at this level.
• Physical representation of the DB on the computer.
• How the data is stored in the database.
• Physical implementation of the DB to achieve optimal run–
time performance and storage space utilization.
- Storage space allocation for data and indexes
- Record description for storage
- Record placement
- Data compression, encryption
Physical level is managed by the OS under the direction of the DBMS.
• Physical representation of the DB on the computer.
• How the data is stored in the database.
• Physical implementation of the DB to achieve optimal run–
time performance and storage space utilization.
- Storage space allocation for data and indexes
- Record description for storage
- Record placement
- Data compression, encryption
Physical level is managed by the OS under the direction of the DBMS.
Logical level or Conceptual level:
This is the middle level of 3-level data abstraction architecture. It describes what data is stored in database.
• The logical structure of the entire database as seen by DBA.
• What data is stored in the database.
• The relationships among the data.
• Complete view of the data requirements of the organization,
independent of any storage consideration.
• Represents:
- entities, attributes, relations
- constraints on data
- semantic information on data
- security, integrity information
Supports each external view: any data available to a user must
be contained in, or derivable from the conceptual level. �
View level or External level:
Highest level of data abstraction. This level describes the user interaction with database system.
• The user’s view of the database.
• Consists of a number of different external views of the DB.
• Describes part of the DB for particular group of users.
• Provides a powerful and flexible security mechanism by
hiding parts of the DB from certain users. The user is not
aware of the existence of any attributes that are missing
from the view.
• It permits users to access data in a way that is customized to
their needs, so that the same data can be seen by different
users in different ways, at the same time.
0 comments