18.5. Distributed Query Processing
In a DDBMS, a query may require data from the databases distributed in more than one site. Some database systems support relation databases whose parts are physically separated. Different relations might reside at different sites, multiple copies of a single relation can be distributed among several sites, or one relation might be partitioned into sub-relations and these sub-relations distributed to multiple sites. In order to evaluate a query issued at a given site, it may be necessary to transfer data between various sites. Therefore, it is important here to optimise on the time required to access such a query, which will be largely comprised of the time spent in transmitting data between sites rather and ...
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