Intermediate files that are transferred to other sites for further processing,Īs well as the final result files that may have to be transferred to the site Theįirst is the cost of transferring data over the net-work. System, several additional factors further complicate query processing. We discussed the issues involved in processingĪnd optimizing a query in a centralized DBMS in Chapter 19. Performed at a central control site, while the last stage is performed locally.Ģ. The first three stages discussed above are This stage is common to all sites in the DDB. This is especially true when the sites are connected through a Since DDBs are connectedīy a net-work, often the communication costs over the network are the most Such as CPU cost, I/O costs, and communication costs. The total cost is a weighted combination of costs Within a fragment query generated by the previous stage. A list ofĬandidate queries can be obtained by permuting the ordering of operations Optimization consists of selecting a strategy from a list of candidates that is closest to optimal. The distributed query on the global schema to separate queries on individualįragments using data distribution and replication information. Separate sites, with some fragments possibly being replicated. It is first normalized, analyzed for semantic errors, simplified, andįinally restructured into an algebraic query. This translation is largely identical to the one performed in a centralizedĭBMS.
Not take into account the actual distribution and replica-tion of data.
This translation is done by referring to the global conceptual schema and does It is then translated into an algebraic query on global relations. Optimization principles using suitable examples.Ī distributed database query is processed in A detailed discussion about optimizationĪlgorithms is beyond the scope of this book.
Finally we discuss a special operation, called a semijoin, which is used to optimize some Processing and then elaborate on the communication costs of processing aĭistributed query. First we discuss the steps involved in query
0 kommentar(er)
