Reverse Key index

There is an option to create index entries as reversed, which is called reverse key indexes. Oracle stores the index entries as their bytes reversed.
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 with Oracle 9i or 10g  Real Application Clusters 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.

Sometimes, using a reverse-key index can make an OLTP Oracle9i or 10g Real Application Clusters application faster. For example, keeping the index of mail messages in an e-mail application: some users keep old messages, and the index must maintain pointers to these as well as to the most recent
There are a few cases where reverse key indexes can help to improve performance. One is in RAC environments. If you have a column populated by an increasing sequence the new entries come to the same blocks when you are inserting rows. If you have many concurrent sessions inserting rows from different RAC instances then you will have a contention for the same index blocks between nodes. If you use reverse key indexes in this case then the new index entries will go to different index blocks and contention will be reduced.
In single instance databases there is also  a case where reverse key indexes can be helpful. If you have a column populated by an increasing sequence, you delete some old rows from the table and you do not perform  range scans on that particular column and you have contention issues on index blocks, reverse key indexes can be considered. The reverse key index will scatter the entries accross different index blocks during insertions and your many concurrent sessions will not have index block contention issues.
In case of deleting some old rows, the blocks from a normal index on that column will have some used and some empty space in them, but they will not be put on the freelist because they are not completely free. That empty space will not be used because the sequence values are always increasing and they will not go to those old blocks because of that. You will be able to use that space for different values with reverse key indexes.

One of the things to be careful about reverse key indexes is that you cannot perform range scans on them. Because the entries are stored as reversed you lose the capability to range scan on that index.