Data can be stored in different computers by fragmenting the whole database into several pieces called fragments. Each piece is stored at a different site.
Fragments are logical data units stored at various sites in a distributed database system.
Advantages of fragmentation
Before we discuss fragmentation in detail, we list four reasons for fragmenting a relation
In general, applications work with views rather than entire relations. Therefore, for data distribution, it seems appropriate to work with subsets of relation as the unit of distribution.
Data is stored close to where it is most frequently used. In addition, data that is ,not needed by' local applications is not stored.
With fragments as the unit of distribution, a transaction can be divided into several sub queries that operate on fragments. This should increase the degree of concurrency, or parallelism, in the system, thereby allowing transactions that can do so safely to execute in parallel.
Data not required by local applications is not stored, and consequently not available to unauthorized users.
Disadvantages of fragmentation
Fragmentation has two primary disadvantages, which we have mentioned previously:
The performance of global application that requires data from several fragments located at different sites may be slower.
Integrity control may be more difficult if data and functional dependencies are fragmented and located at different sites.