Re: snapshot too old issues, first around wraparound and then more.

Hi,

On April 2, 2020 9:36:32 AM PDT, Kevin Grittner <kgrittn@gmail.com> wrote:
>On Wed, Apr 1, 2020 at 7:17 PM Andres Freund <andres@anarazel.de>
>wrote:
>
>> FWIW, with autovacuum=off the query does not get killed until a
>manual
>> vacuum, nor if fewer rows are deleted and the table has previously
>been
>> vacuumed.
>>
>> The vacuum in the second session isn't required. There just needs to
>be
>> something consuming an xid, so that oldSnapshotControl->latest_xmin
>is
>> increased. A single SELECT txid_current(); or such in a separate
>session
>> is sufficient.
>>
>
>Agreed.  I don't see that part as a problem; if no xids are being
>consumed,
>it's hard to see how we could be heading into debilitating levels of
>bloat,
>so there is no need to perform the early pruning.  It would not be
>worth
>consuming any cycles to ensure that pruning happens sooner than it does
>in
>this case.  It's OK for it to happen any time past the moment that the
>snapshot hits the threshold, but it's also OK for it to wait until a
>vacuum
>of the table or until some activity consumes an xid.

The point about txid being sufficient was just about simplifying the reproducer for wrong query results.

Andres
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