Re: "Triggered data change violation", once again

On Wed, 24 Oct 2001, Tom Lane wrote:

> The only reason we do this is to enforce the "triggered data change
> violation" restriction of the spec.  However, I think we've
> misinterpreted the spec.  The code prevents an RI referenced value from
> being changed more than once in a transaction, but what the spec
> actually says is thou shalt not change it more than once per
> *statement*.  We have discussed this several times in the past and
> I think people have agreed that the current behavior is wrong,
> but nothing's been done about it.
> 
> I think all we need to do to implement things correctly is to consider a
> previous event only if both xmin and cmin of the old tuple match the
> current xact & command IDs, rather than considering it on the basis of
> xmin alone.

Are there any things that might update the command ID during the execution
of the statement from inside functions that are being run?  I really don't
understand the details of how that works (which is the biggest reason I
haven't yet tackled some of the big remaining broken stuff in the
referential actions, because AFAICT we need to be able to update a row
that matched at the beginning of the statement, not the ones that match
at the time the triggers run).