Indexed Virtual Column vs Virtual Index vs Invisible Index

Indexed Virtual Column

                It is an alternative to a function based index.
                Add a Virtual Column to the table, and create an index on it.

Eg :
CREATE TABLE EMP
(
EMPNO NUMBER,
ENAME VARCHAR(20),
SAL NUMBER(7,2),
COMM NUMBER(7,2),
TOTALSAL GENERATED ALWAYS AS (SAL +NVL(COMM,0))
);

CREATE INDEX IDX_EMP_TOTALSAL ON EMP(TOTALSAL);

Virtual Index

                You can instruct Oracle to create an index and that will never be used and won't have any extents allocated to it via NOSEGMENT clause

Eg : CREATE INDEX IDX_EMP ON EMP(EMPNO) NOSEGMENT;

                Even though the index is not physically instantiated,  you can instruct Oracle to determine if the index might be used by the optimizer via _USE_NOSEGMENT_INDEXES initialization parameter

Eg : ALTER SESSION SET  "_USE_NOSEGMENT_INDEXES" = true

Invisible Indexes

                Optimizer doesn't use the index when retrieving data for a query. However Insex structure is still maintained as the underlying table has records inserted, updated or deleted. This feature is used if you want to test the viability of an index without impacting existing application code

Eg : CREATE INDEX IDX ON EMP(EMPNO) INVISIBLE ;

Note: Virtual Columns & Invisible Indexes are only available in 11g and higher versions