Feedback-ID: ia2694551:Fastmail
Date: Mon, 24 Jul 2023 12:32:53 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Dean Rasheed <dean.a.rasheed@gmail.com>
Cc: Peter Eisentraut <peter@eisentraut.org>,
	pgsql-hackers@lists.postgresql.org
Subject: Re: cataloguing NOT NULL constraints
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On 2023-Jul-24, Dean Rasheed wrote:

> Hmm, I don't particularly like that approach, because I think it will
> be difficult to cram any additional details into the table, and also I
> don't know whether having multiple not null constraints for a
> particular column can be entirely ruled out.
> 
> I may well be in the minority here, but I think the best way is to
> list them in a separate footer section, in the same way as CHECK
> constraints, allowing other constraint properties to be included. So
> it might look something like:

That's the first thing I proposed actually.  I got one vote down from
Robert Haas[1], but while the idea seems to have had support from Justin
Pryzby (in \dt++) [2] and definitely did from Peter Eisentraut [3], I do
not like it too much myself, mainly because the partition list has a
very similar treatment and I find that one an annoyance.

> and also I don't know whether having multiple not null constraints for
> a particular column can be entirely ruled out.

I had another look at the standard.  In 11.26 (<drop table
constraint definition>) it says that "If [the constraint being removed]
causes some column COL to be known not nullable and no other constraint
causes COL to be known not nullable, then the nullability characteristic
of the column descriptor of COL is changed to possibly nullable".  Which
supports the idea that there might be multiple such constraints.
(However, we could also read this as meaning that the PK could be one
such constraint while NOT NULL is another one.)

However, 11.16 (<drop column not null clause> as part of 11.12 <alter
column definition>), says that DROP NOT NULL causes the indication of
the column as NOT NULL to be removed.  This, to me, says that if you do
have multiple such constraints, you'd better remove them all with that
command.  All in all, I lean towards allowing just one as best as we
can.

[1] https://postgr.es/m/CA+Tgmobnoxt83y1QesBNVArhFm-fLwWkDUyiV84e+psayDwB7A@mail.gmail.com
[2] https://postgr.es/m/20230301223214.GC4268%40telsasoft.com
[3] https://postgr.es/m/1c4f3755-2d10-cae9-647f-91a9f006410e%40enterprisedb.com

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/
“Cuando no hay humildad las personas se degradan” (A. Christie)