drop table base cascade;
drop table log cascade;
create table base (a int, b int);
create view v1 as select a+1 as a1, b+2 as b2 from base;
insert into base values (1,10),(11,13);

create table log(ao int, an int, bo int, bn int);

CREATE OR REPLACE FUNCTION vtrigger()
 RETURNS trigger
 LANGUAGE plpgsql
AS $function$
begin
    raise notice '% % % %', TG_TABLE_NAME, TG_WHEN, TG_OP, TG_LEVEL;
    raise NOTICE 'OLD: %, NEW: %', OLD, NEW;
    UPDATE base SET a = NEW.a1-1, b = NEW.b2-2
      WHERE a = OLD.a1-1 AND b = OLD.b2-2;
    if NOT FOUND then RETURN NULL; end if;
    RETURN NEW;
end;
$function$;

CREATE TRIGGER instead_of_update_trig INSTEAD OF UPDATE ON v1
FOR EACH ROW EXECUTE PROCEDURE vtrigger();

create rule logit as on update to v1 do also
insert into log values(old.a1, new.a1, old.b2, new.b2);

update v1 set a1 = a1-44;