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

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От Kevin Grittner
Тема Re: snapshot too old issues, first around wraparound and then more.
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Msg-id CACjxUsOOMAsAkYy8EEQQNcfzj=VZqJEYwevoP_CD8OofG++kSg@mail.gmail.com
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Ответ на Re: snapshot too old issues, first around wraparound and then more.  (Andres Freund <andres@anarazel.de>)
Ответы Re: snapshot too old issues, first around wraparound and then more.
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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.

--
Kevin Grittner
VMware vCenter Server
https://www.vmware.com/

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