Performance Evaluation and Optimization of Queries in Multi Databases

Modern database systems use a query optimizer to identify query plan to execute declarative SQL queries. Optimization is a mandatory exercise since the difference between the cost of the best plan and a random choice could be in orders of magnitude. The role of query optimizers is especially critical for the decision-support queries featured in data warehousing and data mining applications. For a query on a given database and system configuration the optimizers plan choice is primarily a function of the selectivities of the base relations participating in the query. In this chapter it has been projected towards the cost of sub-query operations on the various sites as well as the benefits achieved in the range of thirty to forty percent for various sub-operations like selection projection and join.