## 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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