Horton

Re: pg_amcheck contrib application

Mark Dilger <mark.dilger@enterprisedb.com>

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From: Mark Dilger <mark.dilger@enterprisedb.com>
To: Robert Haas <robertmhaas@gmail.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: 2021-03-31T17:31:50Z
Lists: pgsql-hackers 

> On Mar 31, 2021, at 10:11 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> 
> On Wed, Mar 31, 2021 at 12:34 AM Mark Dilger
> <mark.dilger@enterprisedb.com> wrote:
>> I'm not looking at the old VACUUM FULL code, but my assumption is that if the xvac code were resurrected, then when a tuple is moved off by a VACUUM FULL, the old tuple and associated toast cannot be pruned until concurrent transactions end.  So, if amcheck is running more-or-less concurrently with the VACUUM FULL and has a snapshot xmin no newer than the xid of the VACUUM FULL's xid, it can check the toast associated with the moved off tuple after the VACUUM FULL commits.  If instead the VACUUM FULL xid was older than amcheck's xmin, then the toast is in danger of being vacuumed away.  So the logic in verify_heapam would need to change to think about this distinction.  We don't have to concern ourselves about that, because VACUUM FULL cannot be running, and so the xid for it must be older than our xmin, and hence the toast is unconditionally not safe to check.
>> 
>> I'm changing the comments back to how you had them, but I'd like to know why my reasoning is wrong.
> 
> Let's start by figuring out *whether* your reasoning is wrong. My
> assumption was that old-style VACUUM FULL would move tuples without
> retoasting. That is, if we decided to move a tuple from page 2 of the
> main table to page 1, we would just write the tuple into page 1,
> marking it moved-in, and on page 2 we would mark it moved-off. And
> that we would not examine or follow any TOAST pointers at all, so
> whatever TOAST entries existed would be shared between the two tuples.
> One tuple or the other would eventually die, depending on whether xvac
> went on to commit or abort, but either way the TOAST doesn't need
> updating because there's always exactly 1 remaining tuple using
> pointers to those TOAST values.
> 
> Your assumption seems to be the opposite, that the TOASTed values
> would be retoasted as part of VF. If that is true, then your idea is
> right.
> 
> Do you agree with this analysis? If so, we can check the code and find
> out which way it actually worked.

Actually, that makes a lot of sense without even looking at the old code.  I was implicitly assuming that the toast table was undergoing a VF also, and that the toast pointers in the main table tuples would be updated to point to the new location, so we'd be unable to follow the pointers to the old location without danger of the old location entries being vacuumed away.  But if the main table tuples get moved while keeping their toast pointers unaltered, then you don't have to worry about that, although you do have to worry that a VF of the main table doesn't help so much with toast table bloat.

We're only discussing this in order to craft the right comment for a bit of code with respect to a hypothetical situation in which VF gets resurrected, so I'm not sure this should be top priority, but I'm curious enough now to go read the old code....

—
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