Function-based indexes provide an efficient mechanism 
for evaluating statements that contain functions in 
their WHERE clauses. The value of the expression is 
computed and stored in the index. When it processes 
INSERT and UPDATE statements, however, Oracle must 
still evaluate the function to process the statement

The upper blocks (branch blocks) of a B-tree index 
contain index data that points to lower-level index
blocks. The lowest level index blocks (leaf blocks) 
contain every indexed data value and a corresponding 
rowid used to locate the actual row. The leaf blocks 
are doubly linked. Indexes in columns containing 
character data are based on the binary values of the 
characters in the database character set.

For a unique index, one rowid exists for each data 
value. For a nonunique index, the rowid is included in 
the key in sorted order, so nonunique indexes are 
sorted by the index key and rowid. Key values 
containing all nulls are not indexed, except for 
cluster indexes. Two rows can both contain all nulls 
without violating a unique index.