Oracle 9i hidden feature: Virtual Index.
Q) What is virtual index:
a) It is hidden index because not showing up or display in data dictionary views.
Q) Does it occupy space from disk?
a) No it not occupy disk space.
Q) What is usage of virtual index?
a) For checking up to tune sql in explain plan.
Q) Why it is hidden?
a) Because it is deployment feature of OEM not from SQL*Plus
For more detail kindly check following link:
Thanks and regards,