What are the alternatives for prefixed indexes in Oracle? You can create an index and define prefix length, so the index will store only first starting symbols from each of column value. It looks like this in MySQL:

CREATE INDEX table_idx ON emp(last_name(8));


Are there an alternative how to do that 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?