如果是unionexpression类，evaluate方法将对两个操作数求值，检查在这两种情况下节点集的结果，然后使用排序合并策略组成联合。

In the case of the UnionExpression class, the evaluate method evaluates the two operands, checks that the result is in both cases a node-set, and then forms the union using a sort-merge strategy.

youdao

Saxon中的常规策略是尽早对标量值求值（字符串、数字、布尔值），而尽可能晚对节点集求值。

The general policy in Saxon is that scalar values (strings, numbers, Booleans) are evaluated as early as possible, while node-set values are evaluated as late as possible.

youdao

Saxon中的常规策略是尽早对标量值求值（字符串、数字、布尔值），而尽可能晚对节点集求值。

The general policy in Saxon is that scalar values (strings, numbers, Booleans) are evaluated as early as possible, while node-set values are evaluated as late as possible.

youdao