Hi,
Good question. From Orbeon Forms' perspective, the code is pretty much the same on all databases, and it's "just" that some databases are much better than others are running XPath expressions on XML data. But I am not sure how good PostgreSQL is at running XPath on XML; the documentation (linked below) doesn't give me any reason to be particularly optimistic, as it doesn't list "performance" as reason to use the `xml` type of the `text` type.
http://www.postgresql.org/docs/9.4/static/datatype-xml.htmlAlex
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