drop table t;
create table t(
    k integer,
    v real
);
create index on t(k);

copy (
    select generate_series(1, 1000000), random()
) to '/tmp/d1';

copy (
    select generate_series(1, 2000000, 2), random()
) to '/tmp/d2';

copy t from '/tmp/d1';

-- 1
drop function f();
create function f() returns trigger language plpgsql as
$$
declare
    rc integer;
begin
    update t set v = v + new.v where k = new.k;
    get diagnostics rc = row_count;
    if rc > 0 then
        return null;
    end if;
    return new;
end;
$$;

create trigger r before insert on t
    for each row execute procedure f() ;

copy t from '/tmp/d2';

-- 2
drop table tmp;
create table tmp(like t);

copy tmp from '/tmp/d2';

update t set v = t.v + tmp.v
    from tmp
    where t.k = tmp.k;

insert into t
    select * from tmp
    where not exists (
        select 1 from t where k = tmp.k
    );