Re: [PoC] XMLCast (SQL/XML X025)

On Sun, Nov 10, 2024 at 1:14 PM Jim Jones <jim(dot)jones(at)uni-muenster(dot)de> wrote:
> rebase.

Hmm, this patch has gotten no responses for 4 months. That's kind of
unfortunate. Sadly, there's not a whole lot that I can do to better
the situation, because I know very little either about XML-related
standards or about how people make use of XML in practice. It's not
that much code, so if it does a useful thing that we actually want, we
can probably figure out how to verify that the code is correct, or fix
it. But I don't know whether it's a useful thing that we actually
want. Syntactically, XMLCAST() looks a lot like CAST(), so one might
ask whether the things that it does can already be accomplished using
CAST(); or whether, perhaps, we have some other existing method for
performing such conversions.

The only thing I found during a quick perusal of the documentation was
XMLTABLE(), which seems a bit baroque if you just want to convert one
value. Is this intended to plug that gap? Is there any other current
way of doing it?

Do we need to ensure some kind of consistency between XMLTABLE() and
XMLCAST() in terms of how they behave? The documentation at
https://www.postgresql.org/docs/current/xml-limits-conformance.html#FUNCTIONS-XML-LIMITS-CASTS
says that "When PostgreSQL maps SQL data values to XML (as in
xmlelement), or XML to SQL (as in the output columns of xmltable),
except for a few cases treated specially, PostgreSQL simply assumes
that the XML data type's XPath 1.0 string form will be valid as the
text-input form of the SQL datatype, and conversely." Unfortunately,
it does not specify what those cases treated specially are, and the
commit that added that documentation text is not the one that added
the underlying code, so I don't actually know where that code is, but
one would expect this function to conform to that general rule.

I emphasize again that if there are people other than the submitter
who are interested in this patch, they should really chime in. This
can't progress in a vacuum.

--
Robert Haas
EDB: http://www.enterprisedb.com