Re: old_snapshot_threshold allows heap:toast disagreement

On Thu, Jul 28, 2016 at 7:29 PM, Andres Freund <andres@anarazel.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
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