Re: Plans for solving the VACUUM problem

Bruce Momjian wrote:
> 
> > > Looking at the previous features you added, like subqueries, MVCC, or
> > > WAL, these were major features that greatly enhanced the system's
> > > capabilities.
> > >
> > > Now, looking at UNDO, I just don't see it in the same league as those
> > > other additions.
> >
> > Hmm hasn't it been an agreement ? I know UNDO was planned
> > for 7.0 and I've never heard objections about it until
> > recently. People also have referred to an overwriting smgr
> > easily. Please tell me how to introduce an overwriting smgr
> > without UNDO.
> 
> I guess that is the question.  Are we heading for an overwriting storage
> manager?

I've never heard that it was given up. So there seems to be
at least a possibility to introduce it in the future.
PostgreSQL could have lived without UNDO due to its no
overwrite smgr. I don't know if avoiding UNDO is possible
to implement partial rollback(I don't think it's easy
anyway). However it seems harmful for the future 
implementation of an overwriting smgr if we would
introduce it.

> I didn't see that in Vadim's list of UNDO advantages, but
> maybe that is his final goal.
> If so UNDO may make sense, but then the
> question is how do we keep MVCC with an overwriting storage manager?
> 

It doesn't seem easy. ISTM it's one of the main reason we
couldn't introduce an overwriting smgr in 7.2.

regards,
Hiroshi Inoue