Stephane,

>
> Without the consideration of the order by, i there any difference
> between ascending and descending index for queries like
>
> select ... where fieldA > 123;
> select ... where fieldA = 123;
> select ... where fieldA < 123;

No difference. The direction of the index matters for ORDER BY and
for MIN (ascending index) or MAX (descending index).

Ignoring the issues of Nulls, a descending index is identical to an
ascending index except that the keys are inverted. Nulls are
complicated in both directions.

Good luck,

Ann