On 2 November 2017 at 22:59, Nico Williams <nico@cryptonector.com> wrote:
> On Thu, Nov 02, 2017 at 03:25:48PM -0700, Peter Geoghegan wrote:
>> Nico Williams <nico@cryptonector.com> wrote:
>> >A MERGE mapped to a DML like this:
>>
>> This is a bad idea. An implementation like this is not at all
>> maintainable.
>
> Assuming the DELETE issue can be addressed, why would this not be
> maintainable?
It would only take one change to make this approach infeasible and
when that happened we would need to revert to the full-executor
version.
One difference that comes to mind is that MERGE doesn't behave the
same way as an UPDATE-join, according to SQL:2011 in that it must
throw an error if duplicate changes are requested. That would be hard
to emulate using a parser only version.
I would call it impressively clever but likely fragile, in this case,
though I encourage more ideas like that in the future.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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