Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On Thu, Jun 16, 2016 at 11:37 AM, Andres Freund <andres@anarazel.de> wrote:
>> If that were really true, why would we not have the problem in
>> current production versions that the toast table could be vacuumed
>> before the heap, leading to exactly the issue you are talking
>> about?
>
> The issue isn't there without the feature, because we (should) never
> access a tuple/detoast a column when it's invisible enough for the
> corresponding toast tuple to be vacuumed away. But with
> old_snapshot_timeout that's obviously (intentionally) not the case
> anymore.  Due to old_snapshot_threshold we'll prune tuples which,
> without it, would still be considered HEAPTUPLE_RECENTLY_DEAD.

Is there really an assumption that the heap and the TOAST heap are
only ever vacuumed with the same OldestXmin value?  Because that seems
like it would be massively flaky.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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