Re: AW: AW: Plans for solving the VACUUM problem

I have relucantly added this UNDO/VACUUM thread to TODO.detail.  People
can review the discussion via a link on the TODO page or in CVS. 
Whenever we resolve this issue, I will gladly remove these emails.


> 
> > > > > 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)
> > 
> > Using WAL as RS data storage is questionable.
> 
> No, I meant the other way around. Move the physical log pages away from WAL 
> files to the "rollback segment" (imho "snapshot area" would be a better name)
> 
> > >     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.
> > 
> > This would be possible only if there was no commit (same SCN)
> > between two modifications.
> 
> I don't think someone else's commit matters unless it touches the same page.
> In that case a reader would possibly need to chain back to an older version 
> inside the snapshot area, and then it gets complicated even in the whole page 
> case. A good concept could probably involve both whole page and change
> only, and let the optimizer decide what to do.
> 
> > But, aren't we too deep on overwriting smgr (O-smgr) implementation?
> 
> Yes, but some understanding of the possibilities needs to be sorted out 
> to allow good decicsions, no ?
> 
> Andreas
> 
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