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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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The McGraw Hill Companies, 2001
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We reconstruct the relation r by taking the union of all fragments; that is, r = r1 r2 rn In our example, the fragments are disjoint By changing the selection predicates used to construct the fragments, we can have a particular tuple of r appear in more than one of the ri In its simplest form, vertical fragmentation is the same as decomposition (see 7) Vertical fragmentation of r(R) involves the de nition of several subsets of attributes R1 , R2 , , Rn of the schema R so that R = R1 R2 Rn Each fragment ri of r is de ned by ri = Ri (r) The fragmentation should be done in such a way that we can reconstruct relation r from the fragments by taking the natural join r = r1
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One way of ensuring that the relation r can be reconstructed is to include the primary-key attributes of R in each of the Ri More generally, any superkey can be used It is often convenient to add a special attribute, called a tuple-id, to the schema R The tuple-id value of a tuple is a unique value that distinguishes the tuple from all other tuples The tuple-id attribute thus serves as a candidate key for the augmented schema, and is included in each of the Ri s The physical or logical address for a tuple can be used as a tuple-id, since each tuple has a unique address To illustrate vertical fragmentation, consider a university database with a relation employee-info that stores, for each employee, employee-id, name, designation, and salary For privacy reasons, this relation may be fragmented into a relation employee-privateinfo containing employee-id and salary, and another relation employee-public-info containing attributes employee-id, name, and designation These may be stored at different sites, again for security reasons The two types of fragmentation can be applied to a single schema; for instance, the fragments obtained by horizontally fragmenting a relation can be further partitioned vertically Fragments can also be replicated In general, a fragment can be replicated, replicas of fragments can be fragmented further, and so on
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The user of a distributed database system should not be required to know either where the data are physically located or how the data can be accessed at the speci c local site This characteristic, called data transparency, can take several forms: Fragmentation transparency Users are not required to know how a relation has been fragmented Replication transparency Users view each data object as logically unique The distributed system may replicate an object to increase either system per-
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
VI Database System Architecture
19 Distributed Databases
The McGraw Hill Companies, 2001
Distributed Transactions
formance or data availability Users do not have to be concerned with what data objects have been replicated, or where replicas have been placed Location transparency Users are not required to know the physical location of the data The distributed database system should be able to nd any data as long as the data identi er is supplied by the user transaction Data items such as relations, fragments, and replicas must have unique names This property is easy to ensure in a centralized database In a distributed database, however, we must take care to ensure that two sites do not use the same name for distinct data items One solution to this problem is to require all names to be registered in a central name server The name server helps to ensure that the same name does not get used for different data items We can also use the name server to locate a data item, given the name of the item This approach, however, suffers from two major disadvantages First, the name server may become a performance bottleneck when data items are located by their names, resulting in poor performance Second, if the name server crashes, it may not be possible for any site in the distributed system to continue to run A more widely used alternative approach requires that each site pre x its own site identi er to any name that it generates This approach ensures that no two sites generate the same name (since each site has a unique identi er) Furthermore, no central control is required This solution, however, fails to achieve location transparency, since site identi ers are attached to names Thus, the account relation might be referred to as site17account, or account@site17, rather than as simply account Many database systems use the internet address of a site to identify it To overcome this problem, the database system can create a set of alternative names or aliases for data items A user may thus refer to data items by simple names that are translated by the system to complete names The mapping of aliases to the real names can be stored at each site With aliases, the user can be unaware of the physical location of a data item Furthermore, the user will be unaffected if the database administrator decides to move a data item from one site to another Users should not have to refer to a speci c replica of a data item Instead, the system should determine which replica to reference on a read request, and should update all replicas on a write request We can ensure that it does so by maintaining a catalog table, which the system uses to determine all replicas for the data item
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