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Reverse Key Indexes

It is very common to have an index based on a value that is constantly changing. For instance, if you use a unique ID for each row the ID number will continually get higher. This results in always adding an index value to one side of the index.
As values are deleted from the index, they are deleted from the other side of the index, as these values are older and less useful. The net effect of these two operations is an unbalanced B*-tree structure, where one side of the index has much more empty space than the other.
You can prevent this imbalance with a reverse key index. As the name implies, a reverse key index stores the index values in reverse order. This causes them to be more evenly distributed through the index structure. As an example, the increasing ID values of 234, 235 and 236 will be stored as 432, 532 and 632, which will be distributed throughout the leaf blocks of the index. Oracle will automatically translate reverse key values on storing and retrieving them.
Of course, you cannot use a reverse key index for sorting.

Oracle Database 12c

Reverse Key Index

A reverse key index is a type of B-tree index that physically reverses the bytes of each index key while keeping the column order. For example, if the index key is 20, and if the two bytes stored for this key in hexadecimal are C1,15 in a standard B-tree index, then a reverse key index stores the bytes as 15,C1.
Reversing the key solves the problem of contention for leaf blocks in the right side of a B-tree index. This problem can be especially acute in an Oracle Real Application Clusters (Oracle RAC) database in which multiple instances repeatedly modify the same block. For example, in an orders table the primary keys for orders are sequential. One instance in the cluster adds order 20, while another adds 21, with each instance writing its key to the same leaf block on the right-hand side of the index. In a reverse key index, the reversal of the byte order distributes inserts across all leaf keys in the index. For example, keys such as 20 and 21 that would have been adjacent in a standard key index are now stored far apart in separate blocks. Thus, I/O for insertions of sequential keys is more evenly distributed. Because the data in the index is not sorted by column key when it is stored, the reverse key arrangement eliminates the ability to run an index range scanning query in some cases. For example, if a user issues a query for order IDs greater than 20, then the database cannot start with the block containing this ID and proceed horizontally through the leaf blocks.