提升性能的一阶谓词，如果重写为索引。
Stage 1 predicates that can improve performance if rewritten as indexable.
例如，编写可索引或一阶谓词，尽量不要编写没有连接谓词（也称为 Cartesian 连接）的查询。
For example, write indexable or stage 1 predicates and avoid writing queries without join predicates (also known as Cartesian join).
XQuery被非正式地设计为可以构造XML文档，而这些XML文档是通过对输入的XML文档进行一阶谓词演算得出的。
Informally, XQuery is designed to be able to construct any XML document that can be computed from input XML documents using the power of the first-order predicate calculus.
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