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。
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.
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.
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