> > > > > You mean it is restored in session that is running the transaction ?
> > >
> > > Depends on what you mean with restored. It first reads the heap page,
> > > sees that it needs an older version and thus reads it from the "rollback segment".
> >
> > So are whole pages stored in rollback segments or just the modified data?
>
> This is implementation dependent. Storing whole pages is much easy to do,
> but obviously it's better to store just modified data.
I am not sure it is necessarily better. Seems to be a tradeoff here.
pros of whole pages:a possible merge with physical log (for first modification of a page after checkpoint there
wouldbe no overhead compared to current since it is already written now)in a clever implementation a page already in
the"rollback segment" might satisfy the modification of another row on that page, and thus would not need any
additionalio.
Andreas