On Mon, Jul 13, 2020 at 9:10 PM Andres Freund <andres@anarazel.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
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