CNF vs DNF

> select * from aa where (bb = 2 and ff = 3) or (bb = 4 and ff = 5);

I've been told that the system restructures these in CNF (conjunctive normal
form)... i.e. the above query turns into:

select * from aa where (bb = 2 or bb = 4) and (ff = 3 or bb = 4) and (bb = 2
or ff = 5) and (ff = 3 or ff = 5);

Much longer and much less efficient, AFAICT. Isn't it more efficient to do a
union of many queries (DNF) than an intersection of many subqueries (CNF)?
Certainly remembering the subqueries takes less memory... Also, queries
already in DNF are probably more common than queries in CNF, requiring less
rewrite.

Can someone clarify this?

Taral