open-source XML-database

Hi all;

currently, I'm working on the following project:
we serve the data about dental products to the web by
storing it in an "XML-portal-server"
http://www.exceloncorp.com/products/portalserver.html
and by then selecting, sorting, and transforming it
with XSLT to HTML.
The XML-DB-server claims to store the data as
preparsed objects,
("... lightening-fast queries, even across extremely
large content sets.")
so the performance is increased: the XSLT-processor
doesn't have to parse several hundred XML-docs each
time a user-request is made from the site.

What would be a free, open-source solution, that
stores large amounts of XML-data in a way that makes
it accessible quickly?

apache + PHP + saxon + ?

Tobi

more info that describes what I'm looking for:
http://www.exceloncorp.com/products/excelon/excelon_dataserver.html
:
"
XML Parser 
XML is parsed and then stored in its parsed format;
that is, its building elements are stored as
individual objects in an XMLStore to support granular
access and reuse of  XML elements. Once XML is stored
in the engine, it can later be retrieved and
manipulated without the need to re-parse. This greatly
improves performance by eliminating the parsing
overhead typical in XML applications.
"

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