Reading the mailing list I just learned that this is possible.

Experimenting shows that (using FB 2.5) this does have limitations.

FB lets me define such indices using computed fields or even select() statements

like

create index idx on buchung computed by (valuta);

where valuta is a computed field

and "show index" does indeed show those indices as existing

but they are never used. If instead I replace "valuta" by its computation like in

create index idx2 on buchung computed by (case when condition then datum else datum+1 end)

this index IS used.

So it seems

1. FB 2.5 does not use a computed index if its expression uses computed fields or select() statements
and probably in more cases

2. FB 2.5 has a bug: It should not accept creation of unusable indices

I absolutely understand that index expressions using computed fields or select() statements
are not possible in the general case and very difficult to implement for most practical
cases (when FB could theoretically analyze dependencies of the expression).

Is that correct so far? Then another bug would be that
http://www.firebirdsql.org/refdocs/langrefupd20-create-index.html
does not mention those restrictions.

--
Wolfgang