Re: old_snapshot_threshold allows heap:toast disagreement

On Tue, Aug 2, 2016 at 9:10 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Sat, Jul 30, 2016 at 8:17 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> On Fri, Jul 29, 2016 at 1:10 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> On Wed, Jul 27, 2016 at 7:26 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>>>
>>> New version attached.
>>
>> +static inline void
>> +InitToastSnapshot(Snapshot snapshot, XLogRecPtr lsn)
>> +{
>> + snapshot->satisfies = HeapTupleSatisfiesToast;
>> + snapshot->lsn = lsn;
>> +}
>>
>> Here, don't you need to initialize whenTaken as that is also used in
>> TestForOldSnapshot_impl() to report error "snapshot too old".
>
> Hmm, yeah. This is actually a bit confusing. We want the "oldest"
> snapshot, but there are three different notions of "oldest":
>
> 1. Smallest LSN.
> 2. Smallest whenTaken.
> 3. Smallest xmin.
>
> Which one do we use?
>

Which ever notion we choose, I think we should use the LSN and
whenTaken from the same snapshot. I think we can choose the snapshot
with smallest xmin and then use the LSN and whenTaken from it. I
think xmin comparison makes more sense because you are already
retrieving smallest xmin snapshot from the registered snapshots.

Whatever value we choose here, I think we can't guarantee that it will
be in sync with the value used for heap. So there is a chance that we
might spuriously raise "snapshot too old" error. I think we should
mentioned this as a caveat in docs [1] where we explain behaviour of
about old_snapshot_threshold.

[1] - https://www.postgresql.org/docs/9.6/static/runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-ASYNC-BEHAVIOR

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com