Is there a commercial RDBMS that allows one to create indices on
views?

I'm asking this partly because of BCNF.

A typical BCNF problem is (where -> is a FD)

A, B->C
C -> B

which becomes the following relations

R_1 [_A_, _C_]
R_2 [_C_, B]

where _X_ denotes an attribute in the primary key

since
R [_A_,_C_,B] is not in 2NF

But there's an additional constraint where the equijoin of R_1 and
R_2
on C would have the candidate key A,B.

Is there a way to express this last constraint (i.e., a candidate key
on a view) in an implementation system?

Cecil Chua