Horton

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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, Tom Lane <tgl@sss.pgh.pa.us>, Stephen Frost <sfrost@snowman.net>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2020-07-14T19:41:59Z
Lists: pgsql-hackers 
Hi,

On 2020-07-14 07:51:27 -0400, Robert Haas wrote:
> On Tue, Jul 14, 2020 at 3:08 AM Peter Eisentraut
> <peter.eisentraut@2ndquadrant.com> wrote:
> > In the meantime, if you're wizard enough to deal with this kind of
> > thing, you could also clone the module from the PG14 tree and build it
> > against older versions manually.
> 
> But what if you are NOT a wizard, and a wizard is giving you
> directions? Then having to build from source is a real pain. And
> that's normally the situation I'm in when a customer has this issue.

The "found xmin ... from before relfrozenxid ..." cases should all be
fixable without needing such a function, and without it making fixing
them significantly easier, no? As far as I understand your suggested
solution, you need the tid(s) of these tuples, right? If you have those,
I don't think it's meaningfully harder to INSERT ... DELETE WHERE ctid =
.... or something like that.

ISTM that the hard part is finding all problematic tuples in an
efficient manner (i.e. that doesn't require one manual VACUUM for each
individual block + parsing VACUUMs error message), not "fixing" those
tuples.

Greetings,

Andres Freund

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.