Re: COPY and Volatile default expressions

On Mon, Apr 15, 2013 at 11:49:42AM -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> > COPY cannot be optimised correctly if we have before triggers or
> > volatile default expressions.
>
> > The multi-insert code detects those cases and falls back to the single
> > row mechanism in those cases.
>
> > There a common class of volatile functions that wouldn't cause
> > problems: any volatile function that doesn't touch the table being
> > loaded and still works correctly when called with alternately ordered
> > data.
>
> > I claim this is a common class, since sequence next_val functions and
> > uuid generators meet that criteria and most common forms of auditing
> > trigger, as well as any other form of data-reformatting trigger.
>
> I don't believe that it's a good idea to consider nextval() to be
> reorderable, so I'm not convinced by your argument here.

We tell people very clearly in the docs and elsewhere that nextval()
guarantees uniqueness and very specifically not ordering, so with
greatest respect, I agree with Simon on its reorderability.

> > What I'd like to do is to invent a new form of labelling that
> > allows us to understand that COPY can still be optimised.
>
> And I don't want to invent impossible-to-verify function attributes
> with such a tiny use-case as this.

Are you referring to the Halting Problem?

Cheers,
David.
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