Distribution Design Issues BY SHREYA MYLA PRESENTATION ID

Distribution Design Issues BY: SHREYA MYLA PRESENTATION ID: 08

Outline: Degree of fragmentation. Correctness Rules of Fragmentation. Allocation alternatives. Information Requirements. Fragmentation and its types.

Degree of fragmentation The extent to which the database should be fragmented is an important decision as that affects the performance of query execution. The degree goes from one extreme, that is, not to fragment at all to the other extreme, to fragment to the level of individual tuples or attributes. Hence, what we need, then , is to find a suitable level of fragmentation that is a compromise between the two extremes.

Correctness of Fragmentation Completeness Decomposition of relation R into fragments R 1, R 2, . . . , Rn is complete if and only if each data item in R can also be found in some Ri Reconstruction If relation R is decomposed into fragments R 1, R 2, . . . , Rn, then there should exist some relational operator ∇ such that R = ∇1≤i≤n. Ri Disjointness If relation R is decomposed into fragments R 1, R 2, . . . , Rn, and data item di is in Rj, then di should not be in any other fragment Rk (k ≠ j ).

Allocation Alternatives Non-replicated Partitioned : each fragment resides at only one site Replicated Fully replicated : each fragment at each site Partially replicated : each fragment at some of the sites Rule of thumb:

Comparison of Replication Alternatives

Information Requirements Four categories: q Database information q Application information q Communication network information q Computer system information

Fragmentation Horizontal Fragmentation (HF) q Primary Horizontal Fragmentation (PHF) q Derived Horizontal Fragmentation (DHF) Vertical Fragmentation (VF) Hybrid Fragmentation (HF)

Horizontal fragmentation partitions a relation along its tuples. Primary horizontal fragmentation of a relation is performed using predicates that are defined on that relation. Derived horizontal fragmentation, on the other hand, is the partitioning of a relation that results from predicates being defined on another relation.

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