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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