Re: Correcting freeze conflict horizon calculation - Mailing list pgsql-hackers

On Fri, May 30, 2025 at 6:10 PM Peter Geoghegan <pg@bowt.ie> wrote:
>
> On Fri, May 30, 2025 at 5:57 PM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
> > I don't see how OldestXmin comes into play with the visibility_cutoff_xid.
>
> Code in heap_page_is_all_visible() (and other place, I guess the other
> one is in pruneheap.c now) have a separate OldestXmin test:
>
> /*
>  * The inserter definitely committed. But is it old enough
>  * that everyone sees it as committed?
>  */
> xmin = HeapTupleHeaderGetXmin(tuple.t_data);
> if (!TransactionIdPrecedes(xmin,
>                            vacrel->cutoffs.OldestXmin))
> {
>     all_visible = false;
>     *all_frozen = false;
>     break;
> }
>
> Once we "break" here, it doesn't matter what visibility_cutoff_xid has
> been set to. It cannot be used for any purpose.

Ah, I see this is done before visibility_cutoff_xid is advanced, so
visibility_cutoff_xid won't end up ever being a value newer than
OldestXmin. So it's not really the newest committed xmin on the page,
it is the newest committed xmin on the page preceding OldestXmin. I
had always been thinking of it as the newest committed xmin on the
page.

- Melanie