It won't give anything back to the operating system, but the oldest
active transaction will move on to the next open transaction and
Firebird can reuse the space occupied by the now obsolete records
rather than have to acquire more space.

Set

> So, if I close this one active transaction, can it affect reduction
> number of unneeded records of old generations?
>
> > Not quite, it means that you have at least one transaction still
> > open and that you've started 170 new transactions since you
> > started this one. It doesn't say anything about whether those
> > remaining 170 transactions are still open or not.
> >
> > > It means, that I have still at about 170 trancations opened(not
> > > committed or rolled back)?