Re: MERGE SQL Statement for PG11

On Tue, Oct 31, 2017 at 5:14 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> I've proposed a SQL Standard compliant implementation that would do
> much more than be new syntax over what we already have.
>
> So these two claims aren't accurate: "radical difference" and "syntax
> sugar over a capability we have".

I think those claims are pretty accurate.

The design of INSERT .. ON CONFLICT UPDATE and the supporting
machinery only work if there is a unique index.  It provides radically
different behavior than what you get with any other DML statement,
with completely novel concurrency behavior, and there is no reasonable
way to emulate that behavior in cases where no relevant unique index
exists.  Therefore, if MERGE eventually uses INSERT .. ON CONFLICT
UPDATE when a relevant unique index exists and does something else,
such as your proposal of taking a strong lock, or Peter's proposal of
doing this in a concurrency-oblivious manner, in other cases, then
those two cases will behave very differently.

And if, in the meantime, MERGE can only handle the cases where there
is a unique index, then it can only handle the cases INSERT .. ON
CONFLICT UPDATE can cover, which makes it, as far as I can see,
syntactic sugar over what we already have.  Maybe it's not entirely -
you might be planning to make some minor functional enhancements - but
it's not clear what those are, and I feel like whatever it is could be
done with less work and more elegance by just extending the INSERT ..
ON CONFLICT UPDATE syntax.

And it does seem to be your intention to only handle the cases which
the INSERT .. ON CONFLICT UPDATE infrastructure can cover, because
upthread you wrote this: "I didn't say it but my intention was to just
throw an ERROR if no single unique index can be identified."  I don't
think anybody's putting words into your mouth here.  We're just
reading what you wrote.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company