Re: [HACKERS] MERGE SQL Statement for PG11 - Mailing list pgsql-hackers

On Sun, Oct 29, 2017 at 4:48 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
> 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.

My whole point is that the way that you seem determined to go on this
is a dead end. I don't think that *anyone* can go improve on what you
come up with if that's based heavily on ON CONFLICT, for the simple
reason that the final user visible design is totally unclear. There is
an easy way to make me shut up - come up with a design for MERGE that
more or less builds on how UPDATE FROM works, rather than building
MERGE on ON CONFLICT. (You might base things like RLS handling on ON
CONFLICT, but in the main MERGE should be like an UPDATE FROM with an
outer join, that can do INSERTs and DELETEs, too.)

The original effort to add MERGE didn't do anything upsert-like, which
Heikki (the GSOC mentor of the project) was perfectly comfortable
with. I'm too lazy to go search the archives right now, but it's
there. Heikki cites the SQL standard.

This is what MERGE *actually is*, which you can clearly see from the
Oracle/SQL Server/DB2 docs. It says this in the first paragraph of
their MERGE documentation. It's crystal clear from their docs -- "This
statement is a convenient way to combine multiple operations. It lets
you avoid multiple INSERT, UPDATE, and DELETE DML statements."

> I'm also very happy to work together on writing the version
> described - you have already done much work in this area.

You seem to want to preserve the ON CONFLICT guarantees at great cost.
But you haven't even defended that based on a high level goal, or a
use case, or something that makes sense to users (describing how it is
possible is another matter). You haven't even tried to convince me.

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

Did you even read them? They are not mere details. They're fundamental
to the semantics of the feature (if you base it on ON CONFLICT). It's
not actually important that you understand them all; the important
message is that generalizing ON CONFLICT has all kinds of terrible
problems.

> 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.

Getting on with *what*, exactly?

In general, I have nothing against an 80% solution, or even a 50%
solution, provided there is a plausible path to a 100% solution. I
don't think that you have such a path, but only because you're tacitly
inserting requirements that no other MERGE implementation has to live
with, that I doubt any implementation *could* live with. Again, I'm
not the one making this complicated, or adding requirements that will
be difficult for you to get in to your v1 -- you're the one doing
that.

The semantics that I suggest (the SQL standard's semantics) will
require less code, and will be far simpler. Right now, I simply don't
understand why you're insisting on using ON CONFLICT without even
saying why. I can only surmise that you think that doing so will
simplify the implementation, but I can guarantee you that it won't.

-- 
Peter Geoghegan


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