Data Fragmentation &Allocation

Data Fragmentation

• Relation may be divided into a number of sub-relations, which are then distributed.

Benefits:

• Usage

• Applications work with views rather than entire relations.

• Efficiency

• Data is stored close to where it is most frequently used.

• Data that is not needed by local applications is not stored.

• Parallelism

• With fragments as unit of distribution, transaction can be divided into several sub queries that operate on fragments.

• Security

• Data not required by local applications is not stored and so not available to unauthorized users.

Types of Fragmentation

• Four types of fragmentation:

• Horizontal.

• Vertical.

• Mixed.

• Derived.

• Other possibility is no fragmentation:

• If relation is small and not updated frequently, may be better not to fragment relation.

Data Allocation:

• Each fragment is stored at site with “optimal” distribution

• Four alternative strategies regarding placement of data:

• Centralized.

• Partitioned (or Fragmented).

• Complete Replication,

• Selective Replication.

• Centralized

• Consists of single database and DBMS stored at one site with users distributed across the network.

• Partitioned

• Database partitioned into disjoint fragments, each fragment assigned to one site.

• Complete Replication

• Consists of maintaining complete copy of database at each site.

• Selective Replication

• Combination of partitioning, replication, and centralization.

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