Bahria University
Department of Computer Sciences
Advanced Databases
Lab08: Using Distributed DB Design Concepts
Objectives
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To understand the fragmentation concept of distributed databases
To understand the replication concept of distributed databases
Fragmentation
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A horizontal fragment of a relation is a subset of the tuples in that relation, specified by a
condition on one or more attributes, where each subset has a certain logical meaning.
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Vertical fragmentation divides a relation “vertically” by columns, keeping only certain attributes
of the relation.
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Mixed (Hybrid) fragmentation is achieved when a relation is fragmented both horizontally and
vertically.
Replication
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Fully replicated distributed database is the one in which the whole database is replicated
(copied) at every site in the distributed system.
Partial replication is when some fragments of the database may be replicated whereas others
may not. The number of copies of each fragment can range from one up to the total number of
sites in the distributed system.
Example of Fragmentation and Replication
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Using the database in Figures 3.5 and 3.6, suppose that the company has three computer sites.
Site 1 is used by company headquarters and accesses all employee and project information
regularly, in addition to keeping track of DEPENDENT information for insurance purposes.
In other words, the whole database in Figure 3.6 can be stored at site 1.
BU, CS Department
Advanced Databases
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Spring 2014
Lab08: Using Distributed DB Design Concepts
Sites 2 and 3 are for departments 5 and 4, respectively. At each of these sites, we expect
frequent access to the EMPLOYEE and PROJECT information for the employees who work in that
department and the projects controlled by that department.
Further, we assume that these sites mainly access the Name, Ssn, Salary, and Super_ssn attributes
of EMPLOYEE.
To determine the fragments to be replicated at sites 2 and 3, first we can horizontally fragment
DEPARTMENT by its key Dnumber.
Then we apply derived fragmentation to the EMPLOYEE, PROJECT, and DEPT_LOCATIONS
relations based on their foreign keys for department number—called Dno, Dnum, and Dnumber,
respectively.
We can vertically fragment the resulting EMPLOYEE fragments to include only the attributes {Name,
Ssn, Salary, Super_ssn, Dno}.
BU, CS Department
Advanced Databases
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Spring 2014
Lab08: Using Distributed DB Design Concepts
We must now fragment the WORKS_ON relation and decide which fragments of WORKS_ON to
store at sites 2 and 3.
We are confronted with the problem that no attribute of WORKS_ON directly indicates the
department to which each tuple belongs. In fact, each tuple in WORKS_ON relates an employee e
to a project P.
We could fragment WORKS_ON based on the department D in which e works or based on the
department D' that controls P.
Exercises
Exercise 1
1. Using the example given above perform the following steps:
a) Create all the example tables and add all the sample data given above.
b) For the fragmentation of sites 2 and 3, draw the tables on a paper along with the
data.
c) Use SQL statements to create views to fulfill the requirements given in the example
to simulate the fragmentation for sites 2 and 3.
 For all views, use the site suffix, for example, for fragmentaion of EMPLOYEE
relation at site 2 use EMPLOEE_2.
 The views should contain the correct data and the correct columns.
d) Upon creation of views, run the SELECT queries on your views to ensure proper
fragmentation.
Note: For proper understanding of the steps to follow, read the details of the example given
in the book (page 898).
2. Is there any replication involved while fulfilling the requirements? If yes, where?