Descending index is function based

Descendin index is implemented as function based index in Oracle. This is documented and discussed.
Nulls may lead to a surprise with a desc unique index.

SQL> create table a(a varchar2(2 char), b varchar2(2 char) );

Table created.

SQL>
SQL> select table_name,column_name,data_type,data_length from user_tab_cols where table_name = 'A';

TABLE_NAME                     COLUMN_NAME                    DATA_TYPE
------------------------------ ------------------------------ --------------------------------------
A                              A                              VARCHAR2
A                              B                              VARCHAR2

SQL>
SQL> create unique index a_idx on a (a,b desc);

Index created.

SQL>
SQL> select table_name,column_name,data_type,data_length from user_tab_cols where table_name = 'A';

TABLE_NAME                     COLUMN_NAME                    DATA_TYPE
------------------------------ ------------------------------ --------------------------------------
A                              A                              VARCHAR2
A                              B                              VARCHAR2
A                              SYS_NC00003$                   RAW

SQL>
SQL> select index_type from user_indexes where index_name = 'A_IDX';

INDEX_TYPE
---------------------------
FUNCTION-BASED NORMAL

SQL>
SQL> select * from user_ind_columns where index_name = 'A_IDX';

INDEX_NAME                     TABLE_NAME
------------------------------ ------------------------------
COLUMN_NAME
----------------------------------------------------------------------------------------------------
COLUMN_POSITION COLUMN_LENGTH CHAR_LENGTH DESC
--------------- ------------- ----------- ----
A_IDX                          A
A
              1             8           2 ASC

A_IDX                          A
SYS_NC00003$
              2            13           0 DESC


SQL>
SQL> select * from USER_IND_EXPRESSIONS where index_name = 'A_IDX';

INDEX_NAME                     TABLE_NAME                     COLUMN_EXPRESSION
------------------------------ ------------------------------ --------------------------------------
A_IDX                          A                              "B"

SQL>
SQL> insert into a values(null,null);

1 row created.

SQL>
SQL> insert into a values(null,null);
insert into a values(null,null)
*
ERROR at line 1:
ORA-00001: unique constraint (KIOS.A_IDX) violated


SQL>
SQL> drop index a_idx;

Index dropped.

SQL>
SQL> create unique index a_idx on a (a,b);

Index created.

SQL>
SQL> insert into a values(null,null);

1 row created.

SQL>
SQL> insert into a values(null,null);

1 row created.