Changes

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 returns only a PUL as a result which is subsequently applied to addressed databases and DOM nodes. A simple expression cannot perform any permanent changes and returns an empty or non-empty sequence.

The most important thing to keep in mind when using XQuery Update is the Pending Update List (PUL). Updating statements are not executed immediately, but are first collected as update primitives within a set-like structure, the PUL. At the end of a query, all update primitives on this list are applied in a strict order after some compatibility tests. 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.