成本是估计值
与预期资源成比例
使用执行语句所需的
有一个特定的计划.优化器
计算访问路径的成本
并根据估计的订单
计算机资源,其中包括
I/O,CPU和内存.
The cost is an estimated value
proportional to the expected resource
use needed to execute the statement
with a particular plan. The optimizer
calculates the cost of access paths
and join orders based on the estimated
computer resources, which includes
I/O, CPU, and memory.
费用较高的序列计划需要
比那些有更多的时间执行
成本更低.使用并行时
计划,但是,资源使用不是
直接与经过时间有关.
Serial plans with higher costs take
more time to execute than those with
smaller costs. When using a parallel
plan, however, resource use is not
directly related to elapsed time.
和
成本代表工作单位或
使用的资源.查询优化器
使用磁盘I/O,CPU使用率和内存
用法作为工作单位.所以,成本
查询优化器使用的表示
磁盘I/O数量的估计
以及使用的CPU和内存量
在执行操作中.这
操作可以扫描表格,
使用访问表中的行
一个索引,将两个表连接在一起,
或对行集进行排序.一个的成本
查询计划是工作单位数
预计将在何时发生
查询被执行及其结果
产生.
The cost represents units of work or
resource used. The query optimizer
uses disk I/O, CPU usage, and memory
usage as units of work. So, the cost
used by the query optimizer represents
an estimate of the number of disk I/Os
and the amount of CPU and memory used
in performing an operation. The
operation can be scanning a table,
accessing rows from a table by using
an index, joining two tables together,
or sorting a row set. The cost of a
query plan is the number of work units
that are expected to be incurred when
the query is executed and its result
produced.
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