指定对于有效的交换，哪些数据元素是必需的，哪些是可选的(基数)？
The specifications of data elements that are required and optional for a valid exchange (the cardinality)?
通常情况下，在外部输入的基数显然不小且对于内部表没有一种有效的访问方法时，优化器会选择合并扫描联接。
Typically, the optimizer chooses merge scan join when the cardinality of the outer input is not obviously small and there is no efficient access method for the inner table.
上面表达式使优化器不能有效分配基数或选择性消耗，可能会反过来影响整个执行计划。
These expressions prevent the optimizer from assigning valid cardinality or selectivity estimates and can in turn affect the overall plan and the join method.
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