select * from unnest(ARRAY[[1,2],[3,4]]);
 unnest
--------
      1
      2
      3
      4
(4 строки)

create or replace function unnest_dim(in_array anyarray)
returns setof anyarray as $$
    declare el in_array%type;
    begin
        foreach el slice 1 in array in_array
        loop
            return next el;
        end loop;
    end;
$$ language plpgsql;

select * from unnest_dim(ARRAY[[1,2],[3,4],[5,6]]);
 unnest_dim
------------
 {1,2}
 {3,4}
 {5,6}
(3 строки)

insert into users (user_id, role1_id, role2_id)
select 15, r[1], r[2]
    from unnest_dim(ARRAY[[1,2],[3,4],[5,6]]) as r;