告别过往的一头雾水 掌握card的标准含义

来源：赛迪网    作者：72933

一直以来，很多人都对card的具体含义一头雾水，参考下文你就会很快得到card的标准含义。

⑴Cost 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.
⑵Card 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 j
oin example, we can see the optimizer expects
there to be 327 rows in EMP and 4 rows in DEPT.
⑶Bytes 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.

至此，我们就可以得出最标准的结论：

card是指计划中这一步所处理的行数。

注释：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。