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help / color / mirror / Atom feedFrom: Bruce Momjian <bruce@momjian.us>
To: David G. Johnston <david.g.johnston@gmail.com>
Cc: dwayne.towell@gmail.com
Cc: pgsql-docs@lists.postgresql.org
Subject: Re: MERGE examples not clear
Date: Fri, 1 Nov 2024 16:21:09 -0400
Message-ID: <ZyU4NaROlZbNUc-q@momjian.us> (raw)
In-Reply-To: <ZRS98kJEt9AZAtpM@momjian.us>
References: <167699245721.1902146.6479762301617101634@wrigleys.postgresql.org>
<CAKFQuwZrP1_hRLT8JXdfX4Bi20-Y3oGPsaKeRVe9bNzngFP+JQ@mail.gmail.com>
<ZRS98kJEt9AZAtpM@momjian.us>
On Wed, Sep 27, 2023 at 07:42:42PM -0400, Bruce Momjian wrote:
> On Tue, Feb 21, 2023 at 08:56:50AM -0700, David G. Johnston wrote:
> > On Tue, Feb 21, 2023 at 8:35 AM PG Doc comments form <noreply@postgresql.org>
> > wrote:
> >
> > The following documentation comment has been logged on the website:
> >
> > Page: https://www.postgresql.org/docs/15/sql-merge.html
> > Description:
> >
> > On this page: https://www.postgresql.org/docs/15/sql-merge.html
> > the first and second examples seems to be contrasted (by "this would be
> > exactly equivalent to the following statement"), however the difference
> > does
> > not seem to related to the stated reason ("the MATCHED result does not
> > change"). It seems like the difference should involve the order of WHEN
> > clauses?
> > Of course, it might be that I don't understand the point, in which case
> > maybe the point could be stated more clearly?
> >
> >
> > Yeah, that is a pretty poor pair of examples. Given that a given customer can
> > reasonably be assumed to have more than one recent transaction the MERGE has a
> > good chance of failing.
> >
> > The only difference between the two is the second one uses an explicit subquery
> > as the source while the first simply names a table. If the subquery had a
> > GROUP BY customer_id that would be a good change explaining that the second
> > query is different because it is resilient in the face of duplicate customer
> > recent transactions.
> >
> > While here...source_alias (...completely hides...the fact that a query was
> > issued). What? Probably it should read (not verified) that it is actually
> > required when the source is a query (maybe tweaking the syntax to match).
>
> The attached patch removes the second example, which doesn't seem to add
> much.
Patch from September 2023 applied.
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EDB https://enterprisedb.com
When a patient asks the doctor, "Am I going to die?", he means
"Am I going to die soon?"
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Cc: bruce@momjian.us, david.g.johnston@gmail.com, dwayne.towell@gmail.com, pgsql-docs@lists.postgresql.org
Subject: Re: MERGE examples not clear
In-Reply-To: <ZyU4NaROlZbNUc-q@momjian.us>
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