
This function prepares and executes the given SQL string, which should be a query expression with
the FOR XML clause at the end of the last term.  The query
is passed the parameters from the params vector, which should have one element for
each ? in the query text, values assigned from left to right.  Consider the
query: select a, b from table where a = ? and b = ?; then the params vector
could reasonably be: vector(1, @myfilter@).

The result set is converted to XML and appended to the string_output.
If the string_output is omitted and the function executes in the context
of a VSP page, the output is sent to the stream going to the user agent.
