UPDATE pg_catalog.pg_class as NO Superuser??

Dear list,

on my system I have multiple user.

I want that the user xy (who is no superuser) can Update a systemtable with:

UPDATE pg_catalog.pg_class SET reltriggers = 0;

With psql I already wrote:

ALTER TABLE pg_catalog.pg_class OWNER TO xy;

and

GRANT ALL PRIVILEGES ON pg_catalog.pg_class TO xy;

I check it with \dS
and see:

                     List of relations
   Schema   |           Name           | Type  |   Owner
------------+--------------------------+-------+-----------
 pg_catalog | pg_aggregate             | table | postgres
 pg_catalog | pg_am                    | table | postgres
 pg_catalog | pg_amop                  | table | postgres
 pg_catalog | pg_amproc                | table | postgres
 pg_catalog | pg_attrdef               | table | postgres
 pg_catalog | pg_attribute             | table | postgres
 pg_catalog | pg_auth_members          | table | postgres
 pg_catalog | pg_authid                | table | postgres
 pg_catalog | pg_autovacuum            | table | postgres
 pg_catalog | pg_cast                  | table | postgres
 pg_catalog | pg_class                 | table | xy


But always if I sent as user xy the
"UPDATE pg_catalog.pg_class SET reltriggers = 0;"-command I get:

"SQL error:

ERROR:  permission denied for relation pg_class"

Why is that? MUST I be a superuser for that?
Is there any way to set that command as user xy?

Thank you very much in advance, Kai





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