Managing risk and uncertainty in query optimization

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The query optimizer, as one of the most important and most complex database system components, is key to the management of large databases. Its major task, query optimization, is to choose the best way to execute a query. Often there are several different ways (execution plans) to perform a given query, which might differ in the indices used, the orders of operators executed, etc., but all lead to the same correct result. The query optimizer generates and examines the candidate execution plans, and makes the decision as to which plan to follow for execution. The quality of the optimizer is very important, because costs of plans can vary dramatically and choosing a bad plan might result in extremely poor performance.