MariaDB / mYSQL Order BY PrimaryKey DESC vs DESC set by Default
Before MySQL 8.0, and MariaDB:
You may use DESC in an index declaration, but it is ignored.  (The PRIMARY KEY is an index.)
You must say DESC in the ORDER BY clause if you want the resultset sorted that way.
For multi-column indexes, all columns in ORDER BY should go the same way (all ASC or all DESC) in order to be able to use an INDEX.  (There are many cases where an index won't be useful for ORDER BY anyway.)
MySQL 8.0:  DESC is honored in INDEXes.