This is a good candidate for a window function. Also note that nulls
already get sorted correctly by the DESC so no need to get 'infinity'
involved, although you could write 'DESC NULLS FIRST' to be explicit about
it.
with x as (select *, row_number() over (partition by id order by
upper(dates) desc, lower(dates) desc) from example)
select id,value,dates from x where row_number = 1;
id | value | dates
----+-------+-------------------------
1 | b | [2010-01-01,)
2 | d | [2010-01-01,2021-01-01)
3 | g | [2013-01-01,)
4 | j | [2010-01-01,2015-01-01)
(4 rows)
Cheers,
Greg