Re: MERGE SQL Statement for PG11

On 28 October 2017 at 22:04, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> On Sat, Oct 28, 2017 at 12:49 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> Nothing I am proposing blocks later work.
>
> Actually, many things will block future work if you go down that road.
> You didn't respond to the specific points I raised, but that doesn't
> mean that they're not real.
>
>> Everything you say makes it clear that a fully generalized solution is
>> going to be many years in the making, assuming we agree.
>
> I think that it's formally impossible as long as you preserve the ON
> CONFLICT guarantees, unless you somehow define the problems out of
> existence. Those are guarantees which no other MERGE implementation
> has ever made, and which the SQL standard says nothing about. And for
> good reasons.
>
>> "The extent to which an SQL-implementation may disallow independent
>> changes that are not significant is implementation-defined”.
>>
>> So we get to choose. I recommend that we choose something practical.
>> We're approaching the 10 year anniversary of my first serious attempt
>> to do MERGE. I say that its time to move forwards with useful
>> solutions, rather than wait another 10 years for the perfect one, even
>> assuming it exists.
>
> As far as I'm concerned, you're the one arguing for an unobtainable
> solution over a good one, not me. I *don't* think you should solve the
> problems that I raise -- you should instead implement MERGE without
> any of the ON CONFLICT guarantees, just like everyone else has.
> Building MERGE on top of the ON CONFLICT guarantees, and ultimately
> arriving at something that is comparable to other implementations over
> many releases might be okay if anyone had the slightest idea of what
> that would look like. You haven't even _described the semantics_,
> which you could do by addressing the specific points that I raised.

I have no objection to you writing a better version than me and if my
work inspires you to complete that in a reasonable timescale then we
all win. I'm also very happy to work together on writing the version
described - you have already done much work in this area.

I don't see any major problems in points you've raised so far, though
obviously there is much detail.

All of this needs to be written and then committed, so I'll get on and
write my proposal. We can then see whether that is an 80% solution or
something less. There are more obvious barriers to completion at this
point, like time and getting on with it.

--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services