> After some tests, I've found I just can do it when nobody else is
> connected.
>
> Is this a common firebird rule or is it possible to change this
> behaviour?

I think you'll notice that you get no errors when you do it..
This means that it actually does get done.. but the change you are making is
not visible to anyone until they all get off and get back on again.

This mechanism actually protects your data against situations where users
are partially thru a transaction and you go and change metadata on them mid
stream. In some cases (perhaps often) this would invalidate what they are
trying to do in their transaction.

Alan