Horton

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

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: 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>, Andres Freund <andres@anarazel.de>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, Stephen Frost <sfrost@snowman.net>
Date: 2020-09-19T20:19:05Z
Lists: pgsql-hackers 
I wrote:
> I was able to partially reproduce whelk's failure here.  I got a
> couple of cases of "cannot freeze committed xmax", which then leads
> to the second NOTICE diff; but I couldn't reproduce the first
> NOTICE diff.  That was out of about a thousand tries :-( so it's not
> looking like a promising thing to reproduce without modifying the test.

... however, it's trivial to reproduce via manual interference,
using the same strategy discussed recently for another case:
add a pg_sleep at the start of the heap_surgery.sql script,
run "make installcheck", and while that's running start another
session in which you begin a serializable transaction, execute
any old SELECT, and wait.  AFAICT this reproduces all of whelk's
symptoms with 100% reliability.

With a little more effort, this could be automated by putting
some long-running transaction (likely, it needn't be any more
complicated than "select pg_sleep(10)") in a second test script
launched in parallel with heap_surgery.sql.

So this confirms the suspicion that the cause of the buildfarm
failures is a concurrently-open transaction, presumably from
autovacuum.  I don't have time to poke further right now.

			regards, tom lane

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.