MariaDB [test]> create or replace table t (a int, key(a desc)) engine=Memory;

Query OK, 0 rows affected (0.040 sec)

MariaDB [test]> show create table t;

+-------+------------------------------------------------------------------------------------------------------------+

| Table | Create Table                                                                                               |

+-------+------------------------------------------------------------------------------------------------------------+

| t     | CREATE TABLE `t` (

  `a` int(11) DEFAULT NULL,

  KEY `a` (`a` DESC)

) ENGINE=MEMORY DEFAULT CHARSET=latin1 |

+-------+------------------------------------------------------------------------------------------------------------+

1 row in set (0.001 sec)

MariaDB [test]> show index in t;

+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+

| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Ignored |

+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+

| t     |          1 | a        |            1 | a           | NULL      |           0 |     NULL | NULL   | YES  | HASH       |         |               | NO      |

+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+

1 row in set (0.001 sec)