horizontal fragmentation, Database Management System

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Horizontal Fragmentation-Horizontal fragmentation groups together the tuples in a relation that are collectively used by the main transactions. A horizontal fragment is produced by specifying a WHERE clause condition that shows a restriction on the tuples in the relation. It can also be described using the Selection operation of the relational algebra.

Example:

Let us demonstrate horizontal fragmentation with the help of an example.

 DEPOSIT (branch-code, account-number, customer-name, balance)

A sample relation instance of the relation DEPOSIT is shown in Figure.

Branch-code

Account number

Customer name

Balance

1101

3050

Suresh

5000

1101

2260

Swami

3360

1102

1170

Swami

2050

1102

4020

Khan

10000

1101

1550

Khan

620

1102

4080

Khan

1123

1102

6390

Khan

7500

                                                              Figure: Sample DEPOSIT relation

Mathematically a fragment may be described as a selection on the global relation R. The reconstruction of the relation R can be taken by taking the union of all fragments.

So let us decay the table in Figure into horizontal fragments. Let us do these fragments on the branch-code as 1101 and 1102

DEPOSIT1 obtained by selection on branch-code as 1101

Branch-code

Account number

Customer name

Balance

1101

3050

Suresh

5000

1101

2260

Swami

3360

1101

1550

Khan

620

DEPOSIT2 obtained by selection on branch- code as 1102

Branch-code

Account number

Customer name

Balance

1102

1770

Swami

2050

1102

4020

Khan

10000

1102

4080

Khan

1123

1102

6390

Khan

7500

 

 

 

 

 

 

 

 

 

                                        Figure: Horizontal fragmentation of relation DEPOSIT

 

The two fragments can be described in relational algebra as:

 

DEPOSIT1 = σ branch-code= 1101 (DEPOSIT)

DEPOSIT2 = σ branch-code= 1102 (DEPOSIT)

These two fragments are given in Figure. Fragment 1 can be stored in the branch whose code is 1101 whereas the second fragment can be stored at branch 1102.

In our example, the fragments are disjoint. Though, by changing the selection predicates used to construct the fragments; we may have overlapping horizontal fragments. This is a type of data replication.


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