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

Hi,

On 2023-01-25 18:45:12 +0300, Aleksander Alekseev wrote:
> Currently we allow self-conflicting inserts for ON CONFLICT DO NOTHING:
> 
> ```
> CREATE TABLE t (a INT UNIQUE, b INT);
> INSERT INTO t VALUES (1,1), (1,2) ON CONFLICT DO NOTHING;
> -- succeeds, inserting the first row and ignoring the second
> ```
> ... but not for ON CONFLICT .. DO UPDATE:
> 
> ```
> INSERT INTO t VALUES (1,1), (1,2) ON CONFLICT (a) DO UPDATE SET b = 0;
> ERROR:  ON CONFLICT DO UPDATE command cannot affect row a second time
> HINT: Ensure that no rows proposed for insertion within the same
> command have duplicate constrained values.
> ```
> 
> Tom pointed out in 2016 that this is actually a bug [1] and I agree.

I don't think I agree with this being a bug.

We can't sensible implement updating a row twice within a statement - hence
erroring out for ON CONFLICT DO UPDATE affecting a row twice. But what's the
justification for erroring out in the DO NOTHING case? ISTM that it's useful
to be able to handle such duplicates, and I don't immediately see what
semantic confusion or implementation difficulty we avoid by erroring out.

It seems somewhat likely that a behavioural change will cause trouble for some
of the uses of DO NOTHING out there.

Greetings,

Andres Freund