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.

Posted Date: 3/13/2013 8:39:45 AM | Location : United States

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