There are a few specialties around XQuery Update that you should know about. In addition to the <b>simple expression</b>, the XQUF adds the <b>updating expression</b> as a new type of expression. An updating expression is able to change the state of a database node, whereas a simple expression cannot perform any permanent changes.
;Pending Update List :The most important thing to keep in mind when using XQuery Update is the Pending Update List (PUL). In contrast to other programming languages, XQUF statements are not executed immediately, but are first collected within a set-like structure, the PUL. At the end of a query, all update primitives on this list are applied after being checked for compatibility. If a conflict exists, an error message is returned and all accessed databases remain untouched (atomicity). For the user this means updates are only visible <b>after</b> the end of a snapshot.