没有必要对每个元素应用该谓词来查看它是否是真，因为处理可能在第三个节点后停止。
It is not necessary to apply this predicate explicitly to every element to see if it is true, since processing can stop after the third node.
必须也为真，因此DB 2查询转换可以添加谓词让它能考虑另一个索引。
Must also be true, so the DB2 query transformation can add that predicate enabling it to consider another index.
方法是最灵活的，因为它让你提供一个谓词，如果它为真，则引发一个参数匹配，如清单6-30所示。
The Is method is the most flexible because it lets you supply a predicate that causes a parameter match if it returns true, as shown in Listing 6-30.
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