Re: MERGE SQL Statement for PG11

On Thu, Nov 2, 2017 at 8:28 AM, Craig Ringer <craig@2ndquadrant.com> wrote:
> On 1 November 2017 at 01:55, Peter Geoghegan <pg@bowt.ie> wrote:
>> The problem here is: Iff the first statement uses ON CONFLICT
>> infrastructure, doesn't the absence of WHEN NOT MATCHED imply
>> different semantics for the remaining updates and deletes in the
>> second version of the query? You've removed what seems like a neat
>> adjunct to the MERGE, but it actually changes everything else too when
>> using READ COMMITTED.
>
> Would these concerns be alleviated by adding some kind of Pg-specific
> decoration that constrained concurrency-safe MERGEs?
>
> So your first statement would be
>
>  MERGE CONCURRENTLY ...
>
> and when you removed the WHEN NOT MATCHED clause it'd ERROR because
> that's no longer able to be done with the same concurrency-safe
> semantics?
>
> I don't know if this would be helpful TBH, or if it would negate
> Simon's compatibility goals. Just another idea.

Yes, that fixes the problem.  Of course, it also turns MERGE
CONCURRENTLY into syntactic sugar for INSERT ON CONFLICT UPDATE, which
brings one back to the question of exactly what we're trying to
achieve here.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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