Re: XMLDocument (SQL/XML X030)

On 01/20/25 14:56, Jim Jones wrote:
> If I understand correctly, the compliant approach would be to always
> treat the input expression as CONTENT:
>
> |PG_RETURN_XML_P(xmlparse((text *) data, XMLOPTION_DOCUMENT, true));|
>
> Is that right?"

Did you mean XMLOPTION_CONTENT?

I think that would be right ... which, to be honest, leaves me
in doubt that there is any substantive work at all for an
XMLDOCUMENT constructor in PostgreSQL to do. Possibly that's why
it wasn't provided originally.

We don't have the XML(SEQUENCE) type, and our single XML type
essentially corresponds to XML(CONTENT(ANY)). Any value that has
already been accepted by the input function should already have
the same serialized form you'd expect from XMLDOCUMENT.

We don't have BY REF and passing XML node trees around, so there's
really no way to notice that you've applied XMLDOCUMENT to wrap
stuff in a 'document node' that isn't visible in the serialized
form that is all we store.

Likewise, I'm not thinking of any way to combine our other XML
constructors / aggregates to produce a result that isn't already
XML(CONTENT(ANY)). (If I could think of a way, I think it would mean
those weren't well-specified.)

Therefore I'm thinking that, given the specifics of our XML support,
a fully conformant and efficient XMLDOCUMENT could be implemented
just by returning its XML argument.

That opens a question of whether it's worth the effort to supply
it at all. Maybe it could reduce the surprise for people coming from
another DBMS and finding it missing, and/or be a placeholder in case
we ever implement enough more of the newer SQL/XML standard for it
to have a real effect.

(In a DBMS implementing the whole XML type hierarchy, you could
have an XML(SEQUENCE) containing a single element node, which
you might think you could easily cast to XML(CONTENT(ANY)) or even
XML(DOCUMENT(ANY)), but the cast would raise 2200U or 2200L, respectively:
the sequence would first need to be transformed into a sequence of
a single document node wrapping the original sequence, which is exactly
what you'd use XMLDOCUMENT to do. And of course you could put RETURNING
CONTENT on that XMLDOCUMENT to accomplish the cast in the same step.)

Regards,
-Chap