Vincent,
> SELECT * FROM TABLE1 ORDER BY X DESC, Y ASC;
>
> (X is a date and Y a varchar)
>
> What would that index be?
>
> Is there a function I can use, to invert x (the date), so that I can
> make a query / index set like :
PostgreSQL's sorting mechanism can use an index either ascending or
descending; it does not make a difference.
However, most of the time ORDER BY does not use an index at all because
a seq scan is faster. Usually, ORDER BY uses an index only when
combined with related criteria and/or the LIMIT clause.
-Josh Berkus