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Introduction to Fake / Virtual / NOSEGMENT Indexes January 11, 2008

Posted by Richard Foote in Fake Indexes, Index Access Path, NOSEGMENT Option, Oracle Cost Based Optimizer, Oracle Indexes, Virtual Indexes.
9 comments

OK, as promised, answer to index fact #5 you may not have known:

“It’s possible to make the CBO reference and use within an execution plan indexes that don’t in actual fact exist”.

Before I start, please note this feature is not officially documented other than the odd Metalink note and requires the setting of an undocumented parameter to work, so please exercise caution.

Fake Indexes (also known as Virtual or Nosegment Indexes) have been around for a long time, since 8i days. They’re used primarily by Oracle Enterprise Manager and its Tuning Pack which has various wizards that can do “what if” type analysis. One of these is the Index Wizard which can kinda “pretend” to create an index and see what the Cost Based Optimizer might do if such an index really existed.

It’s possible to create these Fake indexes manually by using the NOSEGMENT clause when creating an index:

CREATE INDEX Bowie_idx ON Bowie_Table(Ziggy) NOSEGMENT;

This will populate some (but not many) DD related tables but will not actually create an index segment or consume any actual storage. It’s not maintained in any way by DML operations on the parent table and it can’t be altered or rebuilt as can a conventional, “real” index (it will generate an ORA-08114 error if you try to do so). You can analyze or run dbms_stats over the index but the index is not treated as analyzed as such (as can be seen via a 10053 trace).

It’s also only visible to the CBO, if and only if a session has the following parameter set:

ALTER SESSION SET “_use_nosegment_indexes” = true;

The CBO will now consider the index and potentially include it within an execution plan. However, at execution time Oracle can of course not use the index and will revert to the next best thing.

A Fake index is basically an index you have when you don’t really have an index in order to see if it could be useful if it really existed.

This Fake Indexes Demo shows how they work and can be used.

1 down, 7 to go … ;)

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