Thank you for your replies.
The selectivity within the not null part was good, unique constraint so
no duplicates. I did change the null values to some negative impossible
so that the index will work but I have to drop the unique constraint and
to impose unique by trigger.
I was (wrongly) assuming that the null values will be also indexed (to
either start or end of the index).
However, I have another table with the opposite situation. It is a
table for deliveries with a DELIVERED_DATE which is NULL for the
undelivered goods and set to some date for the rest. The usual work is
to select the undelivered (which are few comparing to the constantly
growing delivered items), aka NULL values. In this case the index works.
Is the optimizer transforms the query "WHEN DDATE IS NULL" into
something like "WHEN DDATE is not in index", hence the index will work?
If so, the index asc or desc should not matter

10x

Radu