Date: Thu, 2 Nov 2017 14:16:13 -0500
From: Nico Williams <nico@cryptonector.com>
To: Simon Riggs <simon@2ndquadrant.com>
Cc: Peter Geoghegan <pg@bowt.ie>, Robert Haas <robertmhaas@gmail.com>,
	Michael Paquier <michael.paquier@gmail.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: MERGE SQL Statement for PG11
Message-ID: <20171102191612.GR4496@localhost>
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On Thu, Nov 02, 2017 at 06:49:18PM +0000, Simon Riggs wrote:
> On 1 November 2017 at 18:20, Peter Geoghegan <pg@bowt.ie> wrote:
> > In Postgres, you can avoid duplicate violations with MERGE by using a
> > higher isolation level (these days, those are turned into a
> > serialization error at higher isolation levels when no duplicate is
> > visible to the xact's snapshot).
> 
> So if I understand you correctly, in your view MERGE should just fail
> with an ERROR if it runs concurrently with other DML?
> 
> i.e. if a race condition between the query and an INSERT runs
> concurrently with another INSERT
> 
> We have no interest in making that work?

If you map MERGE to a DML with RETURNING-DML CTEs as I suggested before,
how would that interact with concurrent DMLs?  The INSERT DML of the
mapped statement could produce conflicts that abort the whole MERGE,
correct?

If you want to ignore conflicts arising from concurrency you could
always add an ON CONFLICT DO NOTHING to the INSERT DML in the mapping I
proposed earlier.  Thus a MERGE CONCURRENTLY could just do that.

Is there any reason not to map MERGE as I proposed?

Such an implementation of MERGE wouldn't be online because CTEs are
always implemented sequentially currently.  That's probably reason
enough to eventually produce a native implementation of MERGE, ... or to
revamp the CTE machinery to allow such a mapping to be online.

Nico
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