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执行计划中估计CPU开销的单位是什么? RRS feed

  • 问题

  • 执行计划中估计CPU开销的单位是什么?是秒还是毫秒?
    另外I/O开销的单位是什么?是逻辑读写次数吗?
    运算符开销的单位又是什么?是秒吗?
    还有估计子数的单位是什么?
    2009年12月7日 7:21

答案

  • Here are some explanations:

  • Estimated I/O Cost - these are relative values used for presenting whether a given operation is I/O intensive.  The Query Optimizer assigns these values during parsing and they serve only as a comparative tool to aid in determining where the costs of a given operation lie.  The larger the value, the more cost-intensive the process.
  • Estimated CPU Cost - these are relative values used for presenting whether a given operation is CPU intensive.  The Query Optimizer assigns these values during parsing and they serve only as a comparative tool to aid in determining where the costs of a given operation lie.  The larger the value, the more cost-intensive the process.
  • Estimated Operator Cost - This is the summation of the I/O and CPU estimated costs.  This is the value that is also presented under each operation icon in the graphical execution plan.
  • Estimated Subtree Cost - This is the total of this operation's cost as well as all other operations that preceded it in the query to this point.
  • Estimated Number of Rows - This value is derived based upon the statistics available to the Query Optimizer at the time the execution plan is drafted.  The more current (and the larger the sampling size of the statistics) the more accurate this metric and others will be when compared to the actual data.
  • Estimated Row Size - Also based upon the statistics available at the time of parsing, this value corresponds to how wide the Query Optimizer believes the affected rows to be.  The same rule applies to statistics here as well as with the Estimated Number of Rows - the more current and descriptive the data set used to generate the statistics - the more accurate this value will be in comparison to the actual data.  Good stats also lead to better (more accurate) decisions made by the Query Optimizer when actually processing your queries.
  • Hope this helps.
    Dannol


    Mark as Answer if it helps. This posting is provided "AS IS" with no warranties, confers no rights.
2009年12月7日 9:18