Re: Vacuum ERRORs out considering freezing dead tuples from before OldestXmin - Mailing list pgsql-hackers

On Thu, Jun 20, 2024 at 7:42 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> If vacuum fails to remove a tuple with xmax older than
> VacuumCutoffs->OldestXmin and younger than
> GlobalVisState->maybe_needed, it will ERROR out when determining
> whether or not to freeze the tuple with "cannot freeze committed
> xmax".

One thing I don't understand is why it is okay to freeze the xmax of a
dead tuple just because it is from an aborted update.
heap_prepare_freeze_tuple() is called on HEAPTUPLE_RECENTLY_DEAD
tuples with normal xmaxes (non-multis) so that it can freeze tuples
from aborted updates. The only case in which we freeze dead tuples
with a non-multi xmax is if the xmax is from before OldestXmin and is
also not committed (so from an aborted update). Freezing dead tuples
replaces their xmax with InvalidTransactionId -- which would make them
look alive. So, it makes sense we don't do this for dead tuples in the
common case. But why is it 1) okay and 2) desirable to freeze xmaxes
of tuples from aborted updates? Won't it make them look alive again?

- Melanie