The fn.put function is provided by XQuery Update Facility 1.0 as an extension to the XQuery built-in function library. (The fn: namespace prefix in this section is assumed to be bound to the namespace: http://www.w3.org/2005/xpath-functions.)
fn:put($node as node(), $uri as xs:string) as empty-sequence()
The function stores a document or element to the location specified by $uri. It is normally invoked to create a resource on an external storage system such as a file system or a database. The external effects of fn:put are implementation-defined, since they occur outside the domain of XQuery. The intent is that, if fn:put is invoked on a document node and no error is raised, a subsequent query can access the stored document by invoking fn:doc with the same URI.
See the specification for more details.
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