Horton

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

Ashutosh Sharma <ashu.coek88@gmail.com>

View thread View on postgr.es
From: Ashutosh Sharma <ashu.coek88@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: "Andrey M. Borodin" <x4mmm@yandex-team.ru>, MBeena Emerson <mbeena.emerson@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Andres Freund <andres@anarazel.de>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, Tom Lane <tgl@sss.pgh.pa.us>, Stephen Frost <sfrost@snowman.net>
Date: 2020-08-03T09:05:38Z
Lists: pgsql-hackers 
Hi Robert,

Thanks for the review.

I've gone through all your review comments and understood all of them
except this one:

You really cannot
> modify the buffer like this and then decide, oops, never mind, I think
> I won't mark it dirty or write WAL for the changes. If you do that,
> the buffer is still in memory, but it's now been modified. A
> subsequent operation that modifies it will start with the altered
> state you created here, quite possibly leading to WAL that cannot be
> correctly replayed on the standby. In other words, you've got to
> decide for certain whether you want to proceed with the operation
> *before* you enter the critical section.
>

Could you please explain this point once more in detail? I am not quite
able to understand under what circumstances a buffer would be modified, but
won't be marked as dirty or a WAL won't be written for it.

--
With Regards,
Ashutosh Sharma
EnterpriseDB:http://www.enterprisedb.com

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.