Re: New IndexAM API controlling index vacuum strategies

Hi,

On 2021-04-13 12:59:03 -0700, Peter Geoghegan wrote:
> I agree. Bypassing heap truncation is exactly the kind of thing that
> risks adding significant, unpredictable delay at a time when we need
> to advance relfrozenxid as quickly as possible.
>
> I pushed a trivial commit that makes the failsafe bypass heap
> truncation as well just now.

I'm getting a bit bothered by the speed at which you're pushing fairly
substantial behavioural for vacuum. In this case without even a warning
that you're about to do so.

I don't think it's that blindingly obvious that skipping truncation is
the right thing to do that it doesn't need review. Consider e.g. the
case that you're close to wraparound because you ran out of space for
the amount of WAL VACUUM produces, previously leading to autovacuums
being aborted / the server restarted. The user might then stop regular
activity and try to VACUUM. Skipping the truncation might now make it
harder to actually vacuum all the tables without running out of space.

FWIW, I also don't like that substantial behaviour changes to how vacuum
works were discussed only in a thread titled "New IndexAM API
controlling index vacuum strategies".

Greetings,

Andres Freund