Re: Bug in autovacuum.c?

On Fri, Apr 1, 2011 at 5:48 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> Alvaro Herrera wrote:
>> Excerpts from Bruce Momjian's message of vie abr 01 16:50:29 -0300 2011:
>>
>> > To do the right thing every computation that passes over the xid
>> > wraparound bounary should subtract FirstNormalTransactionId, not just
>> > those that fall in the boundry.  That would prevent the value from going
>> > backward and still allow the mapping you liked;  it isn't worth it, but
>> > that is the right answer.
>>
>> This code is only concerned calculating an immediate the wrap horizon
>> for the autovacuuming run that's about to take place.  If it's wrong in
>> one or three counts doesn't mean much.  Consider what would happen if
>> load was high and it would have taken 100 extra milliseconds to get to
>> that bit: ReadNewTransactionId would have returned a value 3
>> transactions later.  Furthermore, before this value is even used at all
>> for vacuuming, there has to be a whole lot of inter-process signalling,
>> a fork, and a new backend startup.
>>
>> I think this should be left alone.  As you said, it isn't worth it.
>
> Agreed it is not worth it but I think we should at least C comment
> something.   I think at a minimum we should set it to
> FirstNormalTransactionId.

I think you should leave it well enough alone.

> I am not so concerned about this case but about other cases where we are
> computing xid distances across the invalid range.

Such as?

--
Robert Haas
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