Date: Tue, 9 Aug 2022 07:26:22 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: =?iso-8859-1?Q?=C1lvaro?= Herrera <alvherre@alvh.no-ip.org>
Cc: Peter Eisentraut <peter.eisentraut@enterprisedb.com>,
	Amit Langote <amitlangote09@gmail.com>,
	Japin Li <japinli@hotmail.com>, Zhihong Yu <zyu@yugabyte.com>,
	Simon Riggs <simon.riggs@enterprisedb.com>,
	pgsql-hackers@lists.postgresql.org,
	Tomas Vondra <tomas.vondra@enterprisedb.com>,
	Daniel Westermann <dwe@dbi-services.com>,
	Erik Rijkers <er@xs4all.nl>,
	Jaime Casanova <jcasanov@systemguards.com.ec>,
	Andres Freund <andres@anarazel.de>
Subject: Re: support for MERGE
Message-ID: <20220809122621.GC19644@telsasoft.com>
References: <20220801153030.q7wn6phsagjohawm@alvherre.pgsql>
 <20220809094823.xby5o2ww33euubwi@alvherre.pgsql>
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On Tue, Aug 09, 2022 at 11:48:23AM +0200, Álvaro Herrera wrote:
> On 2022-Aug-01, Álvaro Herrera wrote:
> 
> > > >      If <command>MERGE</command> attempts an <command>INSERT</command>
> > > >      and a unique index is present and a duplicate row is concurrently
> > > > +    inserted, then a uniqueness violation error is raised;
> > > > +    <command>MERGE</command> does not attempt to avoid such
> > > > +    errors by evaluating <literal>MATCHED</literal> conditions.
> > > 
> > > This was a portion of a chang that was committed as ffffeebf2.
> > > 
> > > But I don't understand why this changed from "does not attempt to avoid the
> > > error by executing an <command>UPDATE</command>." to "...by evaluating
> > > <literal>MATCHED</literal> conditions."
> > > 
> > > Maybe it means to say "..by re-starting evaluation of match conditions".
> > 
> > Yeah, my thought there is that it may also be possible that the action
> > that would run if the conditions are re-run is a DELETE or a WHEN
> > MATCHED THEN DO NOTHING; so saying "by executing an UPDATE" it leaves
> > out those possibilities.  IOW if we're evaluating NOT MATCHED INSERT and
> > we find a duplicate, we do not go back to MATCHED.
> 
> So I propose to leave it as
> 
>            If <command>MERGE</command> attempts an <command>INSERT</command>
>            and a unique index is present and a duplicate row is concurrently
>            inserted, then a uniqueness violation error is raised;
>            <command>MERGE</command> does not attempt to avoid such
>            errors by restarting evaluation of <literal>MATCHED</literal>
> 	   conditions.

I think by "leave it as" you mean "change it to".
(Meaning, without referencing UPDATE).

> (Is "re-starting" better than "restarting"?)

"re-starting" doesn't currently existing in the docs, so I guess not.
You could also say "starting from scratch the evaluation of MATCHED conditions".

Note that I proposed two other changes in the other thread ("MERGE and parsing
with prepared statements").

  - remove the sentence with "automatic type conversion will be attempted";
  - make examples more similar to emphasize their differences;

-- 
Justin