I am preparing for sql intro test and here is the question

Evaluate this command:
select i.iid, m.name
from inventory i, manufacturer m
where i.mid =m.iid
order by i.description

if all the columns have non-unique indexes, which clause will most likely use an index?

I guess it should be the 'where' clause.

anyone could give me an expl?
BTW, does any one know a good site for preparing ORA tests?

Thanks so much for all your help!