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From: Álvaro Herrera <[email protected]>
To: Bruce Momjian <[email protected]>
Cc: jian he <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Subject: Re: First draft of PG 17 release notes
Date: Thu, 9 May 2024 20:40:00 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 2024-May-09, Bruce Momjian wrote:

> However, I don't see it mentioned as a release note item in the commit
> message or mentioned in our docs. I suppose the release note text would
> be:
> 
> 	Removing a PRIMARY KEY will remove the NOT NULL column specification
> 
> 	Previously the NOT NULL specification would be retained.
> 
> Do we have agreement that we want this release note item?

Yes.  Maybe we want some others too (especially regarding inheritance,
but also regarding the way we handle the constraints internally), and
maybe in this one we want different wording.  How about something like
this:

  Removing a primary key constraint may change the nullability
  characteristic of the columns that the primary key covered.

  If explicit not-null constraints exist on the same column, then they
  continue to be /known not nullable/; otherwise they become /possibly
  nullable/.

This is largely based on the SQL standard's language of a column
descriptor having a "nullability characteristic", which for columns with
not-null or primary key constraints is "known not null".  I don't think
we use those terms anywhere.  I hope this isn't too confusing.

The standard's text on this, in section "4.13 Columns, fields, and
attributes", is

  Every column has a nullability characteristic that indicates whether
  the value from that column can be the null value. A nullability
  characteristic is either known not nullable or possibly nullable.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/

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