"Ann W. Harrison" wrote:

> Ivan Prenosil wrote:
>>
>> MGA ensures that each _transaction_ is atomic (all-or-nothing).
>> Undo-log ensures that each _command_ is atomic.
>
> Not entirely. There are actually a tree of undo logs - each command,
> procedure, trigger, etc. has an undo log

I just did not want to bother others with too much details.
Undo-log can also be used explicitely by SAVEPOINT functions.

> and the main work of the
> transaction has an undo log.

But at this level undo-log is not used always.
It can be disabled either by isc_tpb_no_auto_undo,
or automatically when undo-log gets too big.

Ivan