Re: BUG #17245: Index corruption involving deduplicated entries

On Fri, Oct 29, 2021 at 1:10 AM Alexander Kukushkin <cyberdemn@gmail.com> wrote:
> - Each cluster produces ~3TB of WAL every day (plenty of UPDATEs, about 90% of which are HOT updates).
>
> Corruption was found on all shards, but the list of affected indexes a bit varies from shard to shard.
>
> Database schema:
> - mostly PRIMARY or UNIQUE keys
> - a couple of non-unique btree indexes
> - plenty of foreign keys

You have said elsewhere that you're sure that this isn't the parallel
VACUUM bug, since you know that you didn't run a manual VACUUM, even
once. So I wonder what the issue might be. Since you deleted duplicate
rows from a unique index, there probably weren't very many affected
rows in total. It sounds like a pretty subtle issue to me
(particularly compared to the parallel VACUUM bug, which wasn't all
that subtle when it hit at all).

If I had to guess, I'd guess that it has something to do with the
snapshot scalability work. Specifically, a recently reported issue
involving confusion about the structure of HOT chains during pruning:


https://www.postgresql.org/message-id/flat/20211110192010.ckvfzz352hsba5xf%40alap3.anarazel.de#4c3d9c9988164f5ea3c15999bcf50ce7

Please join in on the other thread if you have anything more to add.

I could easily be wrong about that, though. You upgraded using
pg_upgrade, right? That's certainly a confounding factor here.

-- 
Peter Geoghegan