I don't know whats wrong with your understanding (I don't understand
the calculation of key length), but
http://www.volny.cz/iprenosil/interbase/ip_ib_indexcalculator.htm
agrees with Firebird. Try a length of 64 or less...

HTH,
Set

--- In firebird-support@yahoogroups.com, "martinknappe" wrote:
> Hi Adam, hi Set,
> thanx for your replies. Creating a joint index on asterm, id sounds
> very much like a plan. I just tried to do that with:
>
> CREATE INDEX DICENTRIES_IDX1
> ON DICENTRIES (ASTERM,ID)
>
> And this is what I get:
>
> key size exceeeds implementation restriction for index
> "DICENTRIES_IDX1"
>
> I thought this may have to do with the fact that I am using
> unicode_fss code point ordering on a field that may contain up to 80
> characters and that the combined index of asterm, id may be too
> big..
>
> given the fact that creating an index on asterm alone works fine, i
> thought the extra 4 bytes from field id seem to be what makes this
> combined index too big; so what i did was simply reduce the size of
> field asterm from 80 characters to 70 characters (thereby reducing
> the potential index size by 30) and try again - > and i'm still
> getting this error!
>
> how come it's fine to create an index on an 80-bytes varchar field
> alone, but not, to create a combined one on a 70-bytes varchar field
> and a 4-byte integer? what am i doing wrong here?
>
> thanx very much,
>
> martin
>
> ps: i'm using firebird 1.5