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: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Ashutosh Sharma <ashu.coek88@gmail.com>, Mark Dilger <mark.dilger@enterprisedb.com>, Masahiko Sawada <masahiko.sawada@2ndquadrant.com>, "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>, Stephen Frost <sfrost@snowman.net>
Date: 2020-09-21T20:02:29Z
Lists: pgsql-hackers 
On Mon, Sep 21, 2020 at 2:21 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Right, but what we end up with is that the very same tuple xmin and
> xmax might result in pruning/deletion, or not, depending on whether
> it's part of a HOT chain or not.  That's at best pretty weird, and
> at worst it means that corner-case bugs in other places are triggered
> in only one of the two scenarios ... which is what we have here.

I'm not sure I really understand how that's happening, because surely
HOT chains and non-HOT chains are pruned by the same code, but it
doesn't sound good.

> FWIW, weakening the sanity checks in heap_prepare_freeze_tuple is
> *not* my preferred fix here.  But it'll take some work in other
> places to preserve them.

Make sense.

-- 
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.