> > > > > 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 would be 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 additional io.
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