Re: old_snapshot_threshold allows heap:toast disagreement

On Thu, Jul 28, 2016 at 7:29 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> I think just iterating through the active snapshots would have been
> fine. Afaics there's no guarantee that the first active snapshot pushed
> is the relevant one - in contrast to registered one, which are ordered
> by virtue of the heap.

I think the oldest snapshot has to be on the bottom of the stack; how not?

>> > Hm. Could we perhaps assert that the session has a valid xmin?
>>
>> I don't think so. CLUSTER?
>
> That should have one during any toast lookups afaics - the relevant code
> is
> /* Start a new transaction for each relation. */
> StartTransactionCommand();
> /* functions in indexes may want a snapshot set */
> PushActiveSnapshot(GetTransactionSnapshot());
> /* Do the job. */
> cluster_rel(rvtc->tableOid, rvtc->indexOid, true, stmt->verbose);
> PopActiveSnapshot();
> CommitTransactionCommand();
> right? And
> Snapshot
> GetSnapshotData(Snapshot snapshot)
> {
> ...
>
> if (!TransactionIdIsValid(MyPgXact->xmin))
> MyPgXact->xmin = TransactionXmin = xmin;
> sets xmin.

Hmm, OK, I'll have to check on that.

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