Bidirectional traversal of indexes
The ASC and DESC keywords specify the order in which the database server maintains the index. When you create an index on a column and omit the keywords or specify the ASC keyword, the database server stores the key values in ascending order. If you specify the DESC keyword, the database server stores the key values in descending order.
Ascending order means that the key values are stored in order from
the smallest key to the largest key. For example, if you create an
ascending index on the lname column of the customer table,
last names are stored in the index in the following order: Albertson,
Beatty, Currie
.
Descending order means that the key values are stored in order
from the largest key to the smallest key. For example, if you create
a descending index on the lname column of the customer table,
last names are stored in the index in the following order: Currie,
Beatty, Albertson
.
The bidirectional traversal capability of the database server lets you create just one index on a column and use that index for queries that specify sorting of results in either ascending or descending order of the sort column.