Managing risk and uncertainty in query optimization

查询优化器作为数据库系统中最重要、最复杂的组件之一，是大型数据库管理的关键。它的主要任务是查询优化，即选择执行查询的最佳方式。通常有几种不同的方法(执行计划)来执行给定的查询，它们可能在使用的索引、执行的操作符的顺序等方面有所不同，但都会产生相同的正确结果。查询优化器生成并检查候选执行计划，并决定遵循哪个计划执行。优化器的质量非常重要，因为计划的成本可能差异很大，选择一个糟糕的计划可能会导致极差的性能。

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.