create function f_check_pk ()
   returns event_trigger
   language plpgsql
as $$
declare
   r record;
   l_col name;
begin
   for r in (select * from pg_event_trigger_ddl_commands())
   loop
      if r.command_tag in ('CREATE TABLE', 'ALTER TABLE')
      then
         select a.attname into l_col
           from pg_attribute a
          where a.atttypid::regtype::text in ('numeric')
            and (a.attrelid, a.attnum) in (
                   select c.conrelid, unnest(c.conkey)
                     from pg_constraint c
                    where c.contype = 'p' and c.conrelid = r.objid::regclass
          )
         limit 1;
         if found
         then
            raise exception 'Using numeric column (%) for primary key not allowed', l_col;
         end if;
      end if;
   end loop;
end;
$$;

create event trigger et_check_pk
   on ddl_command_end
   execute procedure f_check_pk();

\echo Testing create table ...
create table t1 (c1 numeric, c2 text, constraint t1_pk primary key (c1,c2));
create table t2 (c1 text, c2 numeric);
\echo Testing alter table ...
alter table t2 add constraint t2_pk primary key (c1, c2);
drop table t2;