>> > Huh? How does a "rollback" work then?
>>
>> Check the archives - how does multiversion architecture work
>> and what is garbage collection.
>> :-)
>
> Ah ok, in that case -- what does the undo-log do?

MGA ensures that each _transaction_ is atomic (all-or-nothing).
Undo-log ensures that each _command_ is atomic.

Undo-log can be used to optimize rollbacks (i.e. that rolled back transaction
cleans after itself and change state of transaction from rolled back
to committed), but it is not necessary.

Ivan
http://www.volny.cz/iprenosil/interbase/