what are Reverse Key Indexes ?

what are Reverse Key Indexes ?
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adixitAsked:
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patelgokulCommented:
Creating a reverse key index, compared to a standard index, reverses the bytes of each column indexed (except the ROWID) while keeping the column order. Such an arrangement can help avoid performance degradation in indexes in an Oracle Parallel Server environment where modifications to the index are concentrated on a small set of leaf blocks. By reversing the keys of the index, the insertions become distributed across all leaf keys in the index.

Using the reverse key arrangement eliminates the ability to run an index range scanning query on the index. Because lexically adjacent keys are not stored next to each other in a reverse-key index, only fetch-by-key or full-index (table) scans can be performed.

Under some circumstances using a reverse-key index can make an OLTP Oracle Parallel Server application faster. For example, keeping the index of mail messages in Oracle Office: some users keep old messages around, and the index must maintain pointers to these as well as to the most recent.

The REVERSE keyword provides a simple mechanism for creating a reverse key index. You can specify the keyword REVERSE along with the optional index specifications in a CREATE INDEX statement:

CREATE INDEX i ON t (a,b,c) REVERSE;


You can specify the keyword NOREVERSE to REBUILD a reverse-key index into one that is not reverse keyed:

ALTER INDEX i REBUILD NOREVERSE;


Rebuilding a reverse-key index without the NOREVERSE keyword produces a rebuilt, reverse-key index. You cannot rebuild a normal index as a reverse key index; you must use the CREATE command instead.

[ Please refer oracle documention for more inforation ]
http://otn.oracle.com/documentation/index.html
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KongAWS Certified Solutions Architect - ProfessionalCommented:
In other words:

Reverse key index stores column values:
1122
5678
3452

As

2211
8765
2543

Advantages:
Used on monotonically increasing column, such as one generated from a sequence, to reduce index height & improve performance on equality searches.

Disadvantages:
Index cannot be used for range scans.
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