Re: Quick idea for reducing VACUUM contention

[Decibel! <decibel(at)decibel(dot)org>]

On Jul 30, 2007, at 8:00 PM, Alvaro Herrera wrote:
> ITAGAKI Takahiro wrote:
> > Alvaro Herrera <alvherre(at)commandprompt(dot)com> wrote:
> > > > I think we might need additional "freezing-xmax" operations to  
> > > > avoid
> > > > XID-wraparound in the first path of vacuum, though it hardly  
> > > > occurs.
> > >
> > > I'm not sure I follow.  Can you elaborate?  Do you mean storing a
> > > separate relfrozenxmax for each table or something like that?
> >
> > We need to work around wraparound of xmax in dead tuples. If we  
> > miss to
> > vacuum them and XID is wrapped, we cannot remove them until the next
> > XID-wraparound, because we treat them to be deleted in the *future*.
>
> Oh, but this should not be a problem, because a tuple is either frozen
> or removed completely -- xmax cannot precede xmin.

What if it's frozen, then deleted, and then we wrap on xmax? Wouldn't  
that make the tuple re-appear?
--
Decibel!, aka Jim Nasby                        decibel(at)decibel(dot)org
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