Horton

Re: recovering from "found xmin ... from before relfrozenxid ..."

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2020-07-14T01:18:10Z
Lists: pgsql-hackers 
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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Fix wrong data table horizon computation during backend startup.

  2. Centralize horizon determination for temp tables, fixing bug due to skew.

  3. pg_surgery: Try to stabilize regression tests.

  4. New contrib module, pg_surgery, with heap surgery functions.

  5. Set cutoff xmin more aggressively when vacuuming a temporary table.

  6. snapshot scalability: Don't compute global horizons while building snapshots.

  7. Introduce vacuum errcontext to display additional information.