Re: [HACKERS] MERGE SQL Statement for PG11

On Mon, Feb 5, 2018 at 7:56 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> I don't think you get to make a unilateral decision to exclude
> features that work everywhere else from the scope of this patch.  If
> there is agreement that those features can be left out of scope, then
> that is one thing, but so far all the commentary about the things that
> you've chosen to exclude has been negative.  Nor have you really given
> any reason why they should be exempt.  You've pointed out that
> parallel query doesn't handle everything (which is certainly true, but
> does not mean that any feature from now and the end of time is allowed
> to exclude from scope whatever seems inconvenient regardless of
> contrary community consensus) and you've pointed out here and
> elsewhere that somebody could go add the features you omitted later
> (which is also true, but misses the general point that we want
> committed patches to be reasonably complete already, not have big gaps
> that someone will have to fix later).

For me, the concern is not really the omission of support for certain
features as such. The concern is that those omissions hint that there
is a problem with the design itself, particularly in the optimizer.
Allowing subselects in the UPDATE part of a MERGE do not seem like
they could be written as a neat adjunct to what Simon already came up
with. If that was possible, Simon probably already would have done it.