本文详细介绍了CBO（成本基础优化器）的基本概念及其在查询计划中的应用。包括card（基数）、cost（成本）和bytes（字节数）三个核心指标的定义与作用。通过具体例子展示了如何评估查询计划的成本，帮助读者理解CBO如何选择最优执行路径。
card是指计划中这一步所处理的行数。 cost指cbo中这一步所耗费的资源，这个值是相对值。 bytes指cbo中这一步所处理所有记录的字节数，是估算出来的一组值。
- Cost： The cost assigned to each step of the query plan by the CBO. The CBO works by generating many different execution paths/plans for the same query and assigns a cost to each and every one. The query plan with the lowest cost wins. In the full outer join example, we can see the total cost for this query is 10.
- Card： Card is short for Cardinality. It is the estimated number of rows that will flow out of a given query plan step. In the full outer join example, we can see the optimizer expects there to be 327 rows in EMP and 4 rows in DEPT.
- Bytes： The size in bytes of the data the CBO expects each step of the plan to return. This is dependent on the number of rows (Card) and the estimated width of the rows.
来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/35489/viewspace-84867/，如需转载，请注明出处，否则将追究法律责任。
转载于:http://blog.itpub.net/35489/viewspace-84867/
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