Therefore, the join cardinality is the product of the cardinalities of two row sets, multiplied by the selectivity of the join predicate.

因此， 连接行集数是两个行集的行集数的乘积，被选择的谓词连接的乘积。

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Cardinality affects join orders and join methods to a large extent.

这个基数会大大影响连接顺序和连接方法。

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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.

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。

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