Is it possible in Firebird to create a single index with ascending and descending columns?
The syntax for index creation does not allow for this:

CREATE [UNIQUE] [ASC[ENDING] | [DESC[ENDING]] INDEX indexname
   ON tablename
   { (&lt;col&gt; [, &lt;col&gt; ...]) | COMPUTED BY (expression) }

&lt;col&gt;  ::=  a column not of type ARRAY, BLOB or COMPUTED BY


As you must set a single order for the entire index, not for each individual column. 

However, 


  It is quite valid to define both an ascending and a descending index
  on the same column or key set.