create function first_nonnull( one anyelement, two anyelement )
    returns anyelement
as $$
begin
    return one;
end
$$
    language plpgsql strict;

create function last_nonnull( one anyelement, two anyelement )
    returns anyelement
as $$
begin
    return two;
end
$$
    language plpgsql strict;

create aggregate first_nonnull( anyelement )
(
    stype = anyelement
  , sfunc = first_nonnull
);

create aggregate last_nonnull( anyelement )
(
    stype = anyelement
  , sfunc = last_nonnull
);


create function first( head anyarray, tail anyelement )
    returns anyarray
as $$
begin
    if head is null or head = '{}' then
        return ARRAY[tail];
    end if;
    return head;
end
$$
    language plpgsql;

create function last( head anyarray, tail anyelement )
    returns anyarray
as $$
begin
    return ARRAY[tail];
end
$$
    language plpgsql;

create function first( head anyarray )
    returns anyelement
as $$
begin
    return head[1];
end
$$
    language plpgsql;

create aggregate first( anyelement )
(
    stype = anyarray
  , initcond = '{}'
  , sfunc = first
  , finalfunc = first
);

create aggregate last( anyelement )
(
    stype = anyarray
  , initcond = '{}'
  , sfunc = last
  , finalfunc = first
);

##</code><code>##

select first(v), first_nonnull(v), last(v), last_nonnull(v)
from ( select NULL::text as v union all
    select 'one' as v union all select 'two' as v ) as q
union all
select first(v), first_nonnull(v), last(v), last_nonnull(v)
from ( select 'one'::text as v union all
    select 'two' as v union all select NULL as v ) as q;

##</code><code>##

 first | first_nonnull | last | last_nonnull
-------+---------------+------+--------------
       | one           | two  | two
 one   | one           |      | two
(2 rows)