Different Query time

I have a view with following code in it

create view test_t as
SELECT TRUNC(DY_DAY) SALE_DATE, FM_POWER_KEY,TO_POWER_KEY,HO_HOUR,MIN(NVL(FMPVH_ACT_VOLUME,0)) PURCH,
(MIN(NVL(TOPVH_ACT_VOLUME,0)) + SUM(LOSS_VOLUME)) SALES FROM sams.act_purchase_vol
HAVING MIN(NVL(FMPVH_ACT_VOLUME,0)) != (MIN(NVL(TOPVH_ACT_VOLUME,0)) + SUM(LOSS_VOLUME))
GROUP BY DY_DAY,FM_POWER_KEY,TO_POWER_KEY,HO_HOUR

if i ran query from above view

select * from test_t where day >= '01-mar-2005'

rows return within 30 sec.

if i ran with
select * from test_t where day = '01-mar-2005'
then it just runs