当优化器创建查询计划时，它将使用下列系统目录信息。
The optimizer USES the following system catalog information as it creates a query plan.
有了这个特性，查询优化器可以在决定访问计划时立即考虑索引。
With this feature, the query optimizer can immediately consider the index when determining an access plan.
当评估查询执行计划时，优化器计算并比较每种连接方法的成本，然后选择要使用的最佳方法。
The optimizer calculates and compares the costs of each join method and selects the best one to use when evaluating query execution plans.
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