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Re: Quick idea for reducing VACUUM contention

From: Decibel! <decibel(at)decibel(dot)org>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Quick idea for reducing VACUUM contention
Date: 2007-07-31 15:09:59
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Lists: pgsql-hackers
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
EnterpriseDB      512.569.9461 (cell)

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