Useless sort by

Hi all,
I have a view v_table defined as following:

select a,b,c,d,e,f
from t_table
sort by a,b,c;

the usage pattern of this view is the following:

select distinct(a) from v_table;
select distinct(b) from v_table where a = "XXX";
select distinct(c) from v_table where a = "XXX" and b = "YYYY";

because of that sort in the view definition the first query above
takes not less than 3 seconds. I have solved this performance issue
removing the sort from the view definition and putting it in the
select reducing the time from > 3secons to < 150ms.

Can not the optimizer take rid of that useless sort on those
kind of queries ?

Regards
Gaetano Mendola