Data Independence
Data
Independence is defined as a property of DBMS that helps you to change the
Database schema (outline) at one level of a database system without requiring
to change the schema (outline) at the next higher level. Data independence
helps you to keep data separated from all programs that make use of it.
There
are two types of data independence: Logical & Physical
A. Logical Data Independence
1. Logical
data independence refers characteristic of being able to change the conceptual
schema without having to change the external schema.
2. Logical
data independence is used to separate the external level from the conceptual
view.
3. If we
do any changes in the conceptual view of the data, then the user view of the
data would not be affected.
4. Logical
data independence occurs at the user interface level.
Logical
Data Independence is the ability to change the conceptual scheme without
changing
1.
External
views
2.
External
API or programs
Any
change made will be absorbed by the mapping between external and conceptual
levels.
When
compared to Physical Data independence, it is challenging to achieve logical
data independence.
Examples of changes under Logical Data
Independence
Due
to Logical independence, any of the below change will not affect the external
layer.
1. Add / Modify
/ Delete a new attribute, entity or relationship is possible without a rewrite
of existing application programs
2. Merging
two records into one
3. Breaking
an existing record into two or more records
B. Physical Data Independence
1. Physical
data independence can be defined as the capacity to change the internal schema without
having to change the conceptual schema.
2. If we
do any changes in the storage size of the database system server, then the
Conceptual structure of the database will not be affected.
3. Physical
data independence is used to separate conceptual levels from the internal
levels.
4. Physical
data independence occurs at the logical interface level.
Physical
data independence helps you to separate conceptual levels from the
internal/physical levels. It allows you to provide a logical description of the
database without the need to specify physical structures. Compared to Logical
Independence, it is easy to achieve physical data independence.
With
Physical independence, you can easily change the physical storage structures or
devices with an effect on the conceptual schema. Any change done would be
absorbed by the mapping between the conceptual and internal levels. Physical
data independence is achieved by the presence of the internal level of the
database and then the transformation from the conceptual level of the database
to the internal level.
Examples of changes under Physical Data
Independence
Due
to Physical independence, any of the below change will not affect the
conceptual layer.
·
Using a
new storage device like Hard Drive or Magnetic Tapes
·
Modifying
the file organization technique in the Database
·
Switching
to different data structures.
·
Changing
the access method.
·
Modifying
indexes.
·
Changes to
compression techniques.
·
Change of
Location of Database from say C drive to D Drive
Difference between Physical and Logical
Data Independence
|
Logica Data Independence |
Physical
Data Independence |
|
Logical
Data Independence is mainly concerned with the structure or changing the data
definition. |
Mainly
concerned with the storage of the data. |
|
It is
difficult as the retrieving of data is mainly dependent on the logical
structure of data. |
It is easy to
retrieve. |
|
Compared to
Physical independence it is difficult to achieve logical data independence. |
Compared to
Logical Independence it is easy to achieve physical data independence. |
|
You need to
make changes in the Application program if new fields are added or deleted
from the database. |
A change in
the physical level usually does not need change at the Application program
level. |
|
Modification at
the logical levels is significant whenever the logical structures of the
database are changed. |
Modifications
made at the internal levels may or may not be needed to improve the
performance of the structure. |
|
Concerned
with conceptual schema |
Concerned
with internal schema |
|
Example:
Add/Modify/Delete a new attribute |
Example:
change in compression techniques, storage devices, etc |
Importance of Data Independence
1. Helps
you to improve the quality of the data.
2. Database
system maintenance becomes affordable
3. Enforcement
of standards and improvement in database security
4. You
don't need to alter data structure in application programs
5. Permit
developers to focus on the general structure of the Database rather than
worrying about the internal implementation
6. It
allows you to improve state which is undamaged or undivided
7. Database
disqualification is vastly reduced.
8. Easily
make modifications in the physical level is needed to improve the performance
of the system.
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