Re: Removing more vacuumlazy.c special cases, relfrozenxid optimizations

Hi,

On 2022-03-31 09:58:18 -0700, Peter Geoghegan wrote:
> On Thu, Mar 31, 2022 at 9:37 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > The only place that appears to access rd_rel->relfrozenxid outside of DDL is
> > heap_abort_speculative().
>
> I wonder how necessary that really is. Even if the XID is before
> relfrozenxid, does that in itself really make it "in the future"?
> Obviously it's often necessary to make the assumption that allowing
> wraparound amounts to allowing XIDs "from the future" to exist, which
> is dangerous. But why here? Won't pruning by VACUUM eventually correct
> the issue anyway?

I don't think we should weaken defenses against xids from before relfrozenxid
in vacuum / amcheck / .... If anything we should strengthen them.

Isn't it also just plainly required for correctness? We'd not necessarily
trigger a vacuum in time to remove the xid before approaching wraparound if we
put in an xid before relfrozenxid? That happening in prune_xid is obviously
les bad than on actual data, but still.

ISTM we should just use our own xid. Yes, it might delay cleanup a bit
longer. But unless there's already crud on the page (with prune_xid already
set, the abort of the speculative insertion isn't likely to make the
difference?

Greetings,

Andres Freund