Re: Quick idea for reducing VACUUM contention

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?
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