Re: [HACKERS] Re: 9.4.1 -> 9.4.2 problem: could not access status of transaction 1

On Tue, Jun 2, 2015 at 5:22 PM, Andres Freund <andres@anarazel.de> wrote:
>> > Hm. If GetOldestMultiXactOnDisk() gets the starting point by scanning
>> > the disk it'll always get one at a segment boundary, right? I'm not sure
>> > that's actually ok; because the value at the beginning of the segment
>> > can very well end up being a 0, as MaybeExtendOffsetSlru() will have
>> > filled the page with zeros.
>> >
>> > I think that should be harmless, the worst that can happen is that
>> > oldestOffset errorneously is 0, which should be correct, even though
>> > possibly overly conservative, in these cases.
>>
>> Uh oh.  That seems like a real bad problem for this approach.  What
>> keeps that from being the opposite of too conservative?  There's no
>> "safe" value in a circular numbering space.
>
> I think it *might* (I'm really jetlagged) be fine because that'll only
> happen after a upgrade from < 9.3. And in that case we initialize
> nextOffset to 0. That ought to safe us?

That's pretty much betting the farm on the bugs we know about today
being the only ones there are.  That seems imprudent.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company