efficiency of group by 1 order by 1

>> Is there a way to eliminate the ugly repeated use of
>> date_trunc('day',endtime)?

>>In this particular case you could say
>>
>> ... GROUP BY 1 ORDER BY 1;

I use a similar SQL, e.g.:
select func(x)....group by func(x) order by func(x)
but my func is rather expensive. Is func(x) evaluated three times
in the above statement? Would it be evaluated only once if I used
select func(x)....group by 1 order by 1

TJ O'Donnell
www.gnova.com