How to do an update with XML column ?

Dear all,
I'm taking the liberty to ask an question. Do you know if there are some
ways in order to update and/or do group by with an XML column ?
For instance,

select id, unnest(xpath(
'//cit:CI_Organisation/cit:name/gco:CharacterString/text()',
CAST(data AS XML),
ARRAY[
ARRAY['cit', 'http://standards.iso.org/iso/19115/-3/cit/2.0'],
ARRAY['gco', 'http://standards.iso.org/iso/19115/-3/gco/1.0'],
ARRAY['mdb','http://standards.iso.org/iso/19115/-3/mdb/2.0'],
ARRAY['cat','http://standards.iso.org/iso/19115/-3/cat/1.0'],
])) as orga_name, changedate, createdate, displayorder,
doctype, extra, popularity, rating, root, schemaid, title, istemplate,
isharvested, harvesturi,
harvestuuid, groupowner, metadata.owner, metadata.source, uuid
from public.metadata
group by orga_name

I obtained :

ERROR: could not identify an equality operator for type xml
LINE 39: group by orga_name
^

SQL state: 42883
Character: 2556

Thanks so much