Re: 9.5 release notes may need ON CONFLICT DO NOTHING compatibility notice for FDW authors

On Mon, May 25, 2015 at 1:28 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> My earlier summary was that the support for multiple constraints has been
> poorly thought through. This is an example of the breakage I have been
> complaining about when we are forced to specify the constraint
> (conflict-target).
>
> This is not just related to FDWs and should not be fixed solely for FDWs.
> This was already an open item for me in 9.5, now even more so.

I agree that the decision to change the current behavior has nothing
to do with FDWs. There is no reason to treat foreign tables
differently to local ones in this regard, which implies that ON
CONFLICT DO UPDATE cannot work with postgres_fdw unless and until
someone invents foreign constraints on foreign tables (I think), or
unless we change our mind generally (for other reasons). So,
certainly, the rationale for mandating (or not mandating) an inference
specification with ON CONFLICT DO UPDATE ought to come from balancing
concerns about safety, compatibility, flexibility, and so on.

I did not mean to imply that your comments were unreasonable/too late.
However, I don't see a lot of demand for changing the behavior. There
is at least some demand for accepting as arbiters multiple unique
constraints (that are not more or less equivalent), from Andres for
example, but that's a different question. It's also something that
could reasonably be added later.

--
Peter Geoghegan