Index called with Union but not with OR clause

Hi All,

I have a select statement

select * from v_func_actual_costs
where parent_project='10478' or proj_pk = '10478'

both the fields parent_project and proj_pk have indexes based on them, but when I ran explain plan on this statement I found that none of the indexes are being called. But, if I make two separate statement and combine them with Union ALL, the indexes are being called. The select statement in this case is

select * from ct_admin.v_func_actual_costs
where parent_project='10478'
union all
select * from ct_admin.v_func_actual_costs
where proj_pk = '10478'

Can anybody help me to find a reason for the same. This is just a part of the query so I cannot use the Union ALL clause.

Thanks in advance

Chitra