Database Reference
In-Depth Information
CO
P
CO
Chicago
P
CO
E
E
P
New York
Los Angeles
CO
Houston
LEGEND
C - Customer File
O - Order File
P - Product File
E - Employee File
Vertical fragment
of File
Horizontal fragment
of File
Full File
Figure 18-15
Sample distributed database system.
For the purpose of our discussion, let us assume a distributed database system
with four sites at New York, Los Angeles, Chicago, and Houston. The database con-
sists of four relations: CUSTOMER, ORDER, PRODUCT, and EMPLOYEE.
Figure 18-15 presents this sample distributed database system and indicates how
the data are distributed. Using the figure, let us examine the levels of desired
transparencies.
Note the four relations and observe how they are fragmented. The CUSTOMER
and ORDER relations are partitioned horizontally, and each site stores the parti-
tions pertinent to it. The EMPLOYEE relation is partitioned vertically. The parti-
tion relating to human resources functions is kept in Los Angeles, and that relating
to payroll is stored in New York. The PRODUCT relation is replicated, and full
copies are stored in New York, Los Angeles, and Chicago. Houston has only occa-
sional sales.
Fragmentation Transparency
You have noted how the database is divided and parts are allocated to the various
sites. However, a user at any site does not know whether the database is stored in
its entirety or is divided into parts. For example, let us say that a user in Chicago
wants a report showing the complete details of all employees. The user query is sub-
mitted as though employee data are stored as whole. You know that all employee
data are not kept together and that the EMPLOYEE relation is partitioned verti-
cally. The DDBMS enables the query to execute by retrieving the information from
both vertical partitions and to produce a consolidated result.
This is fragmentation transparency. The DDBMS hides the complexities of how
the horizontal and vertical partitions are created. It makes the partition scheme
completely transparent to the user.
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