Firebird 2.1.1

Query: select max(ID) from TABLE where FIELD=4
Runs quite slowly. Apparently it scans whole index.
When changed to "min" - it runs immediately.

I have both indices:
create index IDX1 on TABLE(FIELD)
create descending index IDX2 on TABLE(FIELD)

Am I missing something fundamental here? I always thought descending index
should do the thing...

Kirill.