| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: recovering from "found xmin ... from before relfrozenxid ..." |
| Date: | 2020-07-14 01:18:10 |
| Message-ID: | CA+TgmoaeNuCuk+JtAKNT8FGV-Eep+NXzwb-Eqx-+Wvs_Z=7hXg@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Jul 13, 2020 at 9:10 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > What if clog has been truncated so that the xmin can't be looked up?
>
> That's possible, but probably only in cases where xmin actually
> committed.
Isn't that the normal case? I'm imagining something like:
- Tuple gets inserted. Transaction commits.
- VACUUM processes table.
- Mischievous fairies mark page all-visible in the visibility map.
- VACUUM runs lots more times, relfrozenxid advances, but without ever
looking at the page in question, because it's all-visible.
- clog is truncated, rendering xmin no longer accessible.
- User runs VACUUM disabling page skipping, gets ERROR.
- User deletes offending tuple.
- At this point, I think the tuple is both invisible and unprunable?
- Fairies happy, user sad.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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