Horton

Re: pg_amcheck contrib application

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Mark Dilger <mark.dilger@enterprisedb.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Noah Misch <noah@leadboat.com>, Peter Geoghegan <pg@bowt.ie>, Thomas Munro <thomas.munro@gmail.com>, "Andrey M. Borodin" <x4mmm@yandex-team.ru>, Stephen Frost <sfrost@snowman.net>, Michael Paquier <michael@paquier.xyz>, Amul Sul <sulamul@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Andres Freund <andres@anarazel.de>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2021-03-31T17:51:40Z
Lists: pgsql-hackers 
On Wed, Mar 31, 2021 at 1:44 PM Mark Dilger
<mark.dilger@enterprisedb.com> wrote:
> I read "exclusively locks" as meaning it takes an ExclusiveLock, but the code shows that it takes an AccessExclusiveLock.  I think the docs are pretty misleading here, though I understand that grammatically it is hard to say "accessively exclusively locks" or such.  But a part of my analysis was based on the reasoning that if VF only takes an ExclusiveLock, then there must be concurrent readers possible.  VF went away long enough ago that I had forgotten exactly how inconvenient it was.

It kinda depends on what you mean by concurrent readers, because a
transaction that could start on Monday and acquire an XID, and then on
Tuesday you could run VACUUM FULL on relation "foo", and then on
Wednesday the transaction from before could get around to reading some
data from "foo". The two transactions are concurrent, in the sense
that the 3-day transaction was running before the VACUUM FULL, was
still running after VACUUM FULL, read the same pages that the VACUUM
FULL modified, and cares whether the XID from the VACUUM FULL
committed or aborted. But, it's not concurrent in the sense that you
never have a situation where the VACUUM FULL does some of its
modifications, then an overlapping transaction sees them, and then it
does the rest of them.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

Commits

  1. amcheck: Improve some confusing reports about TOAST problems.

  2. amcheck: Reword some messages and fix an alignment problem.

  3. amcheck: fix multiple problems with TOAST pointer validation

  4. amcheck: Remove duplicate XID/MXID bounds checks.

  5. amcheck: Fix verify_heapam's tuple visibility checking rules.

  6. nbtree VACUUM: Cope with buggy opclasses.

  7. Improve pg_amcheck's TAP test 003_check.pl.

  8. Fix a confusing amcheck corruption message.

  9. Doc: add note about how to run the pg_amcheck regression tests.

  10. In pg_amcheck tests, don't depend on perl's Q/q pack code.

  11. pg_amcheck: Keep trying to fix the tests.

  12. pg_amcheck: Try to fix still more test failures.

  13. Try to avoid apparent platform-dependency in IPC::Run

  14. Fix portability issues in pg_amcheck's 004_verify_heapam.pl.

  15. Try to fix compiler warnings.

  16. Add pg_amcheck, a CLI for contrib/amcheck.

  17. Refactor and generalize the ParallelSlot machinery.

  18. Remove old-style VACUUM FULL (which was known for a little while as