About the intention and backtracking of the Oracle descending index

oracle| Index
The descending index is essentially the FBI, and its specific definition can be queried by user_ind_expressions or dba_ind_expressions.
Similarly, a descending index can be used only under the CBO.

Connected to oracle9i Enterprise Edition release 9.2.0.4.0
Connected as Eygle


Sql> CREATE TABLE T as select * from Dba_users;

Table created

Sql> CREATE index Idx_username_desc on T (username desc);

Index created


Sql> Select Index_name,table_name,index_type from user_indexes where table_name= ' T ';

INDEX_NAME table_name Index_type
------------------------------ ------------------------------ ---------------------------
Idx_username_desc T function-based NORMAL


Sql> Select Column_name,column_position,descend from User_ind_columns
2 where table_name= ' T ';

column_name Column_position Descend
------------------------------ --------------- -------
sys_nc00013$ 1 DESC

Sql>


Sql> SELECT * from user_ind_expressions where table_name= ' T ';

INDEX_NAME table_name Column_expression column_position
------------------------------ ------------------------------ ------------------------------ ---------------
Idx_username_desc T "USERNAME" 1