本文解析了SQL执行计划中Cost、Card及Bytes等概念的具体含义及其计算方式。这些指标用于评估SQL查询操作的成本，并通过成本基的方法来估算所涉及的数据行数和字节数。值得注意的是，这些值仅作为内部计算使用，不同SQL之间的这些值不具备直接对比的意义。
Cost: the value of this column does not have any particular unit of measurement; It is merely a weighted value used to compute costs of execution plans.
Card: estimated by the cost-based approach of the number of rows accessed by the operation.
Bytes: Estimated by the cost-based approach of the number of bytes accessed by the operation.
above values are not valueable to be referred or compared if they are generated by issuing different SQLs.
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转载于:http://blog.itpub.net/24123206/viewspace-692453/
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