Per https://groups.google.com/group/liftweb/browse_thread/thread/83955de9798f4d0c?pli=1

It would be useful to have the ability to map an xmltype column from oracle (also supported by postgres).

Additionally, the ability to search by xpath taking advantage of the xmlexists('xpath' passing <column>) syntax would be even more useful.

This is a broad tutorial of using the feature and most importantly the xmlindex that comes with 11gR2 (which indexes all xpaths in all documents).
http://www.oracle.com/technetwork/testcontent/xmldb-086889.html
http://download.oracle.com/docs/cd/B10501_01/appdev.920/a96620/xdb04cre.htm

postgres - http://developer.postgresql.org/pgdocs/postgres/datatype-xml.html
derby - http://db.apache.org/derby/docs/10.2/ref/rrefsqljtypexml.html

I haven't personally used derby or postgres, but the concept seems similar.