Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)

On Thu, Jan 29, 2026 at 8:39 AM Kirill Reshke <reshkekirill(at)gmail(dot)com> wrote:
>
> Thanks Alexander!
> This is a good and detailed report, I was able to reproduce this.

Thanks to both of you for looking into it!

> I have added some logs to my copy of postgres with your patch and I
> think problem causing this test to fail is this sequence:
>
> 1) Autovacuum starts, does its deeds, and acquiring xid = 118518

So, in this scenario, is the issue that autovacuum runs before vacuum
freeze? If so, we can change the table DDL to:

create table test_vac_unmodified_heap(a int) with (autovacuum_enabled = false);

which would prevent the autovacuum from running.

Unless there is some other way for one of the other tests to hold
OldestXmin back to before the xid of the insert. But I don't see how.

> 2) insert into test_vac_unmodified_heap values (1); executes and
> commits with xid = 118519 (from my log)
> 3) vacuum freeze starts and computes cutoff xid = 118518, because
> oldest xmin is 118518 from (1)
>
> *and we cannot freeze tuple*

- Melanie