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This requires a number of extensions to the XQuery 3.0 and XQUF processing model.[A statement is a grammatical construct which does not return anything (no XDM, no PUL) but can have side-effects.Therefore a simple "store" or "put" function, applied to the result of such transformation, could seem sufficient to achieve any kind of database update operation. In practice this would be neither very natural, convenient, nor very efficient (such an approach requires storing back the whole document and makes optimizing very difficult).So as we will see the little complexity added by XQuery Update seems quite worth the effort.Subsequent expressions can see the effects of the expressions that came before them.XQuery Scripting Extension Proposal extends XQuery 3.0, enabling it to serve as a scripting language in order to satisfy XQuery Scripting Requirements.The XQuery Update Facility is a relatively small extension of the XQuery language which provides convenient means of modifying XML documents or data.
Because of their side effects, sequential expressions must be evaluated in evaluation order, as defined in this specification.
The term snapshot is defined in XQuery Update Facility as follows: [A snapshot is a scope within which expressions are evaluated with respect to a fixed XDM instance, accessible through the dynamic context's bindings, and updates are held pending.] A snapshot is terminated by invocation of the upd:apply Updates operation.
Unlike XQUF, XQSX permits an XQuery program to contain more than one snapshot.
Statements have to be executed in the evaluation order defined in XQSX.] XQSX introduces the following statements: XQSX builds on top of XQuery 3.0 and XQuery Update 1.0, so that it keeps the same constraints about an expression being simple (if it returns an empty PUL), updating (if it returns an empty XDM) or vacuous (if both are empty).
This specification takes into account the fact that the XQUF specification might evolve and allow an updating expression to return both a PUL and an XDM.
We use the terminology of "non-updating" for simple expressions.