Re: [PATCH] Make ON CONFLICT DO NOTHING and ON CONFLICT DO UPDATE consistent

Hi,

On 2023-01-26 13:07:08 +0300, Aleksander Alekseev wrote:
> > It *certainly* can't be right to just continue with the update in heap_update,
>
> I see no reason why. What makes this case so different from updating a
> tuple created by the previous command?

To me it's a pretty fundamental violation of how heap visibility works. I'm
quite sure that there will be problems, but I don't feel like investing the
time to find a reproducer for something that I'm ready to reject on principle.

> > as you've done. You'd have to skip the update, not execute it. What am I
> > missing here?
>
> Simply skipping updates in a statement that literally says DO UPDATE
> doesn't seem to be the behavior a user would expect.

Given that we skip the update in "UPDATE", your argument doesn't hold much
water.

> > I think this'd completely break triggers, for example, because they won't be
> > able to get the prior row version, since it won't actually be a row ever
> > visible (due to cmin=cmax).
> >
> > I suspect it might break unique constraints as well, because we'd end up with
> > an invisible row in part of the ctid chain.
>
> That's a reasonable concern, however I was unable to break unique
> constraints or triggers so far:

I think you'd have to do a careful analysis of a lot of code for that to hold
any water.

I continue to think that we should just reject this behavioural change.

Greetings,

Andres Freund