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The _use_nosegment_indexes parameter and nosegment keyword

Oracle Tips by Burleson Consulting

Oracle has a special hidden "nosegment" clause to the create index syntax that allows you to create an index definition without actually creating the index.

A virtual index is a "fake" index whose definition exists in the data dictionary, but has no index tress association.  It is used by Oracle developers to test whether a specific index is going to use useful without having to use the disk space associated with the "real" index.  The hidden parameter _use_nosegment_indexes is used by Quest tools and is also use in the Oracle Tuning pack.

Creating a virtual index is easy:

create index

If you own the extra cost Oracle Tuning Pack, you will note the "virtual index wizard" area.  According to Oracle, virtual indexes will help you determine how the Oracle cost-based SQL optimizer (CBO) will evaluate and use the potential index.

Because the virtual index is a fake index, Oracle will never be able to use it, but you can use the hidden _use_nosegment_indexes parameter to evaluate execution plans for virtual indexes:

set autotrace on explain;
alter session set "_use_nosegment_indexes" = true;

Session altered.

SQL> select ename from emp where ename = 'KING';

1 rows selected.

---------- ----------------------------------------
                 SELECT STATEMENT()
                  NESTED LOOPS()
EMP                TABLE ACCESS(FULL)


I cannot really see a use for this undocumented Oracle features except in cases where you wanted to evaluate how the CBO will change execution plans on very large tables.


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