On Tue, Aug 2, 2016 at 9:10 AM, Robert Haas <robertmh...@gmail.com> wrote:
> On Sat, Jul 30, 2016 at 8:17 AM, Amit Kapila <amit.kapil...@gmail.com> wrote:
>> On Fri, Jul 29, 2016 at 1:10 AM, Robert Haas <robertmh...@gmail.com> wrote:
>>> On Wed, Jul 27, 2016 at 7:26 PM, Andres Freund <and...@anarazel.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] - 

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

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