
> > > > > 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.

Andreas

Tue, 29 May 2001 09:35:01 +0200
Zeugswetter Andreas SB  <ZeugswetterA@wien.spardat.at>
AW: AW: Plans for solving the VACUUM problem