Message-ID: <0c98e0da0df5780f05e3eb598bb84955450a74f8.camel@cybertec.at>
Subject: Re: COALESCE documentation
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Peter Eisentraut <peter@eisentraut.org>,
	=?UTF-8?Q?Navr=C3=A1til=2C_Ond=C5=99ej?=
	 <onavratil@monetplus.cz>, pgsql-docs@lists.postgresql.org
Date: Wed, 03 Jul 2024 11:11:51 +0200
In-Reply-To: <62f888dc-1ee5-48cf-b76c-6a75dcfa8b10@eisentraut.org>
References: 
	<CAEELDaqnn5GayihUUhnKLR1_fegb-_b2i+4_dP4U192uK7j9vw@mail.gmail.com>
	 <62f888dc-1ee5-48cf-b76c-6a75dcfa8b10@eisentraut.org>
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On Wed, 2024-07-03 at 11:00 +0200, Peter Eisentraut wrote:
> On 02.07.24 12:45, Navr=C3=A1til, Ond=C5=99ej wrote:
> > as per documentation=20
> > <https://www.postgresql.org/docs/current/functions-conditional.html#FUN=
CTIONS-COALESCE-NVL-IFNULL>
> >  > The |COALESCE|=C2=A0function returns the first of its arguments that=
 is=20
> > not null. Null is returned only if all arguments are null.
> >=20
> > This is not exactly true. In fact:
> > The |COALESCE|=C2=A0function returns the first of its arguments that *i=
s=20
> > distinct* *from *null. Null is returned only if all arguments *are not=
=20
> > distinct from* null.
> >=20
> > See my stack overflow question here=20
> > <https://stackoverflow.com/questions/78691097/postgres-null-on-composit=
e-types>.
> >=20
> > Long story short
> >=20
> > > select coalesce((null, null), (10, 20)) as magic; |
> >=20
> > returns
> >=20
> > > magic ------- (,) (1 row)|
> >=20
> > However, this is true:
> >=20
> > > select (null, null) is null;|
>=20
> I think this is actually a bug in the implementation, not in the=20
> documentation.  That is, the implementation should behave like the=20
> documentation suggests.

You are right.  I find this in the standard:

COALESCE (V1, V2) is equivalent to the following <case specification>:

CASE WHEN V1
IS NOT NULL THEN
V1 ELSE
V2 END

That would mean that coalesce(ROW(1,NULL), ROW(2,1)) should return
the second argument.  Blech.  I am worried about the compatibility pain
such a bugfix would cause...

Yours,
Laurenz Albe