Re: BUG: Postgres 14 + vacuum_defer_cleanup_age + FOR UPDATE + UPDATE

On Sat, Feb 4, 2023 at 2:57 AM Andres Freund <andres@anarazel.de> wrote:
> Is there a good way to make breakage in the page recycling mechanism
> visible with gist? I guess to see corruption, I'd have to halt a scan
> before a page is visited with gdb, then cause the page to be recycled
> prematurely in another session, then unblock the first? Which'd then
> visit that page, thinking it to be in a different part of the tree than
> it actually is?

Yes. This bug is similar to an ancient nbtree bug fixed back in 2012,
by commit d3abbbeb.

> which clearly doesn't seem right.
>
> I just can't quite judge how bad that is.

It's really hard to judge, even if you're an expert. We're talking
about a fairly chaotic scenario. My guess is that there is a very
small chance of a very unpleasant scenario if you have a GiST index
that has regular page deletions, and if you use
vacuum_defer_cleanup_age. It's likely that most GiST indexes never
have any page deletions due to the workload characteristics.

-- 
Peter Geoghegan