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: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Robert Haas <robertmhaas@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>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, Stephen Frost <sfrost@snowman.net>
Date: 2020-09-21T21:01:09Z
Lists: pgsql-hackers 
Hi,

On 2020-09-21 16:40:40 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> >> I think to move forward, we need to figure out what the freezing
> >> behavior ought to be for temp tables.  We could make it the same
> >> as it was before a7212be8b, which'd just require some more complexity
> >> in vacuum_set_xid_limits.  However, that negates the idea that we'd
> >> like VACUUM's behavior on a temp table to be fully independent of
> >> whether concurrent transactions exist.  I'd prefer to allow a7212be8b's
> >> behavior to stand, but then it seems we need to lobotomize the error
> >> check in heap_prepare_freeze_tuple to some extent.
> 
> > I think that's an argument for what I suggested elsewhere, which is that
> > we should move the logic for a different horizon for temp tables out of
> > vacuum_set_xid_limits, and into procarray.
> 
> But procarray does not seem like a great place for
> table-persistence-dependent decisions either?

That ship has sailed a long long time ago though. GetOldestXmin() has
looked at the passed in relation for a quite a while, and even before
that we had logic about 'allDbs' etc.  It doesn't easily seem possible
to avoid that, given how intimately that's coupled with how snapshots
are built and used, database & vacuumFlags checks etc.

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.