insert into marks (mk_guid, mk_st_guid, mk_mark5)
  select mk_guid, mk_st_guid, mk_mark5
    from dblink('dbname=main',
      'select mk_guid, mk_st_guid, mk_mark5 from marks limit 10')
  as t (mk_guid character(36),
         mk_st_guid character varying(36),
         mk_mark5 integer)
  on conflict (mk_guid) do update set
    mk_guid = excluded.mk_guid,
    mk_st_guid = excluded.mk_st_guid,
    mk_mark5 = excluded.mk_mark5;