Re: XMLDocument (SQL/XML X030)

On 01/22/25 13:41, Robert Treat wrote:
> So even if we are following the spec (which I think technically we may
> not be),

There are definite ways in which we're not following the SQL/XML spec,
which we document in an appendix[1]. The one that matters here is that
we just have a single XML type instead of the hierarchy of them in the
spec, and ours corresponds to what the spec calls XML(CONTENT(ANY)).

With that divergence from the spec understood, I don't see any new
divergence in providing an XMLDOCUMENT function that just returns
its argument. That's the correct result to return for anything that's
a valid value of our XML type to begin with.

> if no other database implements it the way we are

There may be other systems that don't implement it at all, for which
case I don't see any compatibility issue created because we have it
and they do not.

There may be systems that implement the SQL/XML type hierarchy more
completely than we do, so that it would be possible for their
XMLDOCUMENT to be called with an XML(SEQUENCE) argument, or with
RETURNING SEQUENCE, both of which are things that can't happen
in PostgreSQL. I don't see a problem in that either, as long as
theirs produces results matching ours in the RETURNING CONTENT,
passed an XML(CONTENT...) argument case.

If another system produces results that differ, in that restricted
domain corresponding to ours, I'd say something's nonconformant in
that implementation. In my opinion, that would only be a problem
for us if the system in question is an 800 lb gorilla and has many
users relying on the differing behavior.

Regarding the patch itself: I wonder if there is already, somewhere
in the code base, a generic fmgr identity function that the pg_proc
entry could point to. Or is there a strong project tradition in favor
of writing a dedicated new one to use here? I'm not sure it's critical
to have a version that tests USE_LIBXML or reports it's unsupported,
because without support I doubt there's any way to pass it a non-null
XML argument, and if declared STRICT it won't be called for a null one
anyway.

Regards,
-Chap

[1] https://www.postgresql.org/docs/17/xml-limits-conformance.html