Re: [HACKERS] MERGE SQL Statement for PG11

On 28 October 2017 at 20:39, Peter Geoghegan <pg@bowt.ie> wrote:
> On Sat, Oct 28, 2017 at 3:10 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
>> SQL:2011 specifically states "The extent to which an
>> SQL-implementation may disallow independent changes that are not
>> significant is implementation-defined”, so in my reading the above
>> behaviour would make us fully spec compliant. Thank you to Peter for
>> providing the infrastructure on which this is now possible for PG11.
>>
>> Serge puts this very nicely by identifying two different use cases for MERGE.
>
> MERGE benefits from having a join that is more or less implemented in
> the same way as any other join. It can be a merge join, hash join, or
> nestloop join. ON CONFLICT doesn't work using a join.
>
> Should I to take it that you won't be supporting any of these
> alternative join algorithms? If not, then you'll have something that
> really isn't comparable to MERGE as implemented in Oracle, SQL Server,
> or DB2. They *all* do this.
>
> Would the user be able to omit WHEN NOT MATCHED/INSERT, as is the case
> with every existing MERGE implementation? If so, what actually happens
> under the hood when WHEN NOT MATCHED is omitted? For example, would
> you actually use a regular "UPDATE FROM" style join, as opposed to the
> ON CONFLICT infrastructure? And, if that is so, can you justify the
> semantic difference for rows that are updated in each scenario
> (omitted vs. not omitted) in READ COMMITTED mode? Note that this could
> be the difference between updating a row when *no* version is visible
> to our MVCC snapshot, as opposed to doing the EPQ stuff and updating
> the latest row version if possible. That's a huge, surprising
> difference. On top of all this, you risk live-lock if INSERT isn't a
> possible outcome (this is also why ON CONFLICT can never accept a
> predicate on its INSERT portion -- again, quite unlike MERGE).
>
> Why not just follow what other systems do? It's actually easier to go
> that way, and you get a better outcome. ON CONFLICT involves what you
> could call a sleight of hand, and I fear that you don't appreciate
> just how specialized the internal infrastructure is.
>
>> Now, I accept that you might also want a MERGE statement that
>> continues to work even if there is no unique constraint, but it would
>> need to have different properties to the above. I do not in any way
>> argue against adding that.
>
> Maybe you *should* be arguing against it, though, and arguing against
> ever supporting anything but equijoins, because these things will
> *become* impossible if you go down that road. By starting with the ON
> CONFLICT infrastructure, while framing no-unique-index-support as work
> for some unspecified future release, you're leaving it up to someone
> else to resolve the problems. Someone else must square the circle of
> mixing ON CONFLICT semantics with fully generalized MERGE semantics.
> But who?

Nothing I am proposing blocks later work.

Everything you say makes it clear that a fully generalized solution is
going to be many years in the making, assuming we agree.

"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.

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


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