它们一次只能用于一个文档，且返回文档片断(可能为空)。
They only apply to one document at a time and may return a (possibly empty) fragment of a document.
唯一潜在的缺点是，当前文档片断还没有得到充分利用，开发者可能不熟悉此技术。
The only potential drawback is that the practice of using document fragments is currently underused and some team members may not be familiar with the technique.
DB 2使用XSCAN操作符来遍历XML文档树，如需要，还将计算谓词和提取文档片断和值。
DB2 USES the XSCAN operator to traverse XML document trees and, if needed, to evaluate predicates and extract document fragments and values.
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