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.