Varun

• Sep 11th, 2005

 

1. These are permanent and continue to exist unless we drop them.

2. Their creation will not affect existing and new sessions. Only sessions marked for Virtual Index usage will become aware of their existence.

3. Such indexes will be used only when the hidden parameter "_use_nosegment_indexes" is set to true.

4. The Rule based optimizer did not recognize Virtual Indexes when I
tested, however, CBO recognizes them. In all of my examples, I have used CBO. However, I did not carry out intensive testing in RBO and you may come across exceptions to this view.

5. Dictionary view DBA_SEGMENTS will not show an entry for Virtual Indexes. The table DBA_INDEXES and DBA_OBJECTS will have an entry for them in Oracle 8i; in Oracle 9i onwards, DBA_INDEXES no longer show Virtual Indexes.

6. Virtual Indexes cannot be altered and throw a "fake index" error!

7. Virtual Indexes can be analyzed, using the ANALYZE command or DBMS_STATS package, but the statistics cannot be viewed (in Oracle 8i, DBA_INDEXES will not show this either). Oracle may be generating artificial statistics and storing it somewhere for referring it later.
Creating Virtual Index
Creating a Virtual Index can be achieved by using the NOSEGMENT clause with the CREATE INDEX command.

senthilora

 Profile Answers by senthilora 

• Sep 24th, 2007

 

And.. It does not store any data value in it unlike normal index do.Queries will not get benefitted. This can be used only for analysis.