What are the alternatives for prefixed indexes in Oracle?
You can create an index on an expression (which you cannot do in MySQL):

CREATE INDEX table_idx ON emp(substr(last_name,1,8));


But that won't help anything unless you also use that expression in the where clause, e.g. 

where substr(last_name,1,8) = 'foobar'


What exactly is the usage of an index in MySQL that doesn't index the whole value?