Horton

Re: pg_amcheck contrib application

Mark Dilger <mark.dilger@enterprisedb.com>

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From: Mark Dilger <mark.dilger@enterprisedb.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Noah Misch <noah@leadboat.com>, Robert Haas <robertmhaas@gmail.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: 2021-03-15T19:20:01Z
Lists: pgsql-hackers 

> On Mar 15, 2021, at 11:57 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> 
> Do we have a strong enough lock on
> the table under examination to be sure that autovacuum couldn't remove
> a dead toast entry before we reach it?

The main table and the toast table are only locked with AccessShareLock.  Each page in the main table is locked with BUFFER_LOCK_SHARE.  Toast is not checked unless the tuple passes visibility checks verifying the tuple is not dead.

>  But this would only be an
> issue if we are trying to check validity of toasted fields within
> known-dead tuples, which I would argue we shouldn't, since lock or
> no lock there's no guarantee the toast entry is still there.

It does not intentionally check toasted fields within dead tuples.  If that is happening, it's a bug, possibly in the visibility function.  But I'm not seeing a specific reason to assume that is the issue.  If we still see the complaint on tern or hornet after committing the patch to turn off autovacuum, we will be able to rule out the theory that the toast was removed by autovacuum.

—
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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