XML Query Optimizations in the Presence of Side Effects
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Recently, several initiatives are proposing to extend XQuery not only
with updates, but also with more general forms of side effects. Some
of the extensions are essential to facilitate the development of
complex XML-based Web applications (grid resource allocation, XML
scripting etc.)

This is an important departure from traditional database languages, in
which there is a strong separation between updates and queries. One of
the reason for this separation is that lack of support for
side-effects in database algebras and compilers. The main contribution
of our work is the design and implementation of a database algebra
with side effects. This algebra is used to compile several recently
proposed XML languages, including the W3C XQuery Update facility, and
XQueryP, a procedural extension to XQuery. We show how some of the
most important query optimizations (such as join detection and query
unnesting) can be recovered.