Database Reference
In-Depth Information
it. Data Independence has two types that
Physical Independence
and
Logical Independen-
ce
.
2.6.1 Physical Data Independence
The ability to modify the physical file structure of a database without disrupting existing
users and application program is known as
physical data independence
. As shown earlier in
figure, it is the separation of the physical layer from the logical layer that provides physical
data independence in a DBMS. The measure, sometimes called the
degree
of physical data
independence, is how much change can be made in the file system without impacting the
logical layer. In systems without data independence, even the slightest change to the way
data was stored required the programmers to make changes to every computer program that
used the data.
All modern computer systems have some degree of physical data independence. However,
on most personal systems, the user must still remember where they placed the file so they
can locate it when they need it again. DBMSs expand greatly on the physical data inde-
pendence provided by the computer system in that they allow database users to access data-
base objects ( tables in a relational DBMS) without having to reference the physical data
files in any way. The DBMS catalog keeps track of where the objects are physically stored.
Here are some examples of physical changes that may be made in a data-independent man-
ner:
Moving a database data file from one device to another or one directory to another
Splitting or combining database data files
Renaming database files
Moving a database object from one data file to another
Adding new database objects or data files
2.6.2 Logical Data Independence
The ability to change the logical layer with out disrupting existing users and application
program is called logical data independence. It is the transformation between the logical
layer and the external layer that provides logical data independence. It is important to know
that most logical changes also involve a physical change. For example, a new database ob-
ject (such as a table in a relational DBMS) cannot be added without physically storing the
data somewhere; hence, there is a corresponding change in the physical layer. Moreover,
deletion of objects in the logical layer will cause anything that uses those objects to fail but
should not affect anything else.
