Re: Nested transactions: low level stuff

Sorry I have a basic question.
Was there any consensus we would introduce nested transactions
(or savepoints) in the way currently discussed ?

regards,
Hiroshi Inoue

Manfred Koizar wrote:
>
> On Wed, 19 Mar 2003 13:00:07 -0500, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
> wrote:
> >Manfred Koizar <mkoi-pg(at)aon(dot)at> writes:
> >> And if the change is lost, it can
> >> be redone by the next backend visiting the tuple.
> >
> >Not if the subtransaction log state has been removed as no longer
> >needed.
>
> But this problem is not triggered by a tuple that has its xmin changed
> by a visitor and then looses that change again. We'd have the same
> problems with tuples that have never been visited (*). So we must
> make sure that pg_subtrans segments are not discarded as long as they
> are needed.
>
> (*) I guess your argument is: VACUUM makes sure that all tuples have
> been visited before it discards pg_subtrans segments.
>
> With my 4-state-proposal VACUUM can decide whether a pg_subtrans
> segment is still needed by only looking at pg_clog.
>
> > I think a WAL entry will be essential.
>
> I'm still in doubt, but it's moot (see below).
>
> >I think we'd be a lot better off to design this so that we don't need to
> >alter heap tuple xmin values...
>
> If Vadim remembers correctly we cannot safely change xmin, unless we
> want to grab a write lock. Ok, we'll not change xmin and we'll not
> set the commit bit before xmin is visible to all if xmin is a
> subtransaction. We can always add this performance hack later, if
> someone finds a safe implementation ...
>
> Servus
> Manfred
>
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--
Hiroshi Inoue
http://www.geocities.jp/inocchichichi/psqlodbc/