Re: Bug in autovacuum.c?

On Thu, Mar 31, 2011 at 4:16 PM, Bruce Momjian <bruce@momjian.us> wrote:
> Robert Haas wrote:
>> On Thu, Mar 31, 2011 at 2:59 PM, Bruce Momjian <bruce@momjian.us> wrote:
>> > Bruce Momjian wrote:
>> >> > Yeah, I think this change would have the effect of moving the freeze
>> >> > limit by one (or two?) counts. ?Given the moving nature of values
>> >> > returned by ReadNewTransactionId this would probably have no practical
>> >> > effect. ?Still, the code as is seems more natural to me (Tom wrote this
>> >> > bit IIRC, not me).
>> >>
>> >> I am now thinking the code is correct --- it maps values from 0 to
>> >> FirstNormalTransactionId into the top of the (unsigned) xid range.
>> >> Unless someone objects, I will add a C comment about this behavior so
>> >> future readers are not confused.
>> >
>> > OK, now I think it is wrong. ? :-)
>> >
>> > The effect is to map max xid + 1 to max xid -
>> > FirstNormalTransactionId(3) + 1, which makes the xid look like it is
>> > going backwards, less than max xid --- not good.
>>
>> The XID space is *circular*.
>
> Right but you would think that as the xid moves forward, the caculation
> of how far back to vacuum should move only forward.  In this case,
> incrementing the xid by one would cause the vacuum horizon to move
> backward by two.

I don't see how that would happen.   The XID immediately preceding
FirstNormalTransactionId is 2^32-1, and that's exactly what this
calculation produces.

--
Robert Haas
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