Re: First draft of PG 17 release notes - Mailing list pgsql-hackers

On Thu, May  9, 2024 at 08:40:00PM +0200, Álvaro Herrera wrote:
> 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.

Yes, it was confusing, partly because it is using wording we don't use,
and partly because it is talking about what can go into the column,
rather than the visible column restriction NOT NULL.  I also think "may"
is too imprecise.

How about:

    Removing a primary key will remove a column's NOT NULL constraint
    if the constraint was added by the primary key
    
    Previously such NOT NULL constraints would remain after a primary
    key was removed.  A column-level NOT NULL constraint would not be
    emoved.

Here is the PG 16 output:

    CREATE TABLE test ( x INT CONSTRAINT test_pkey PRIMARY KEY );
                    Table "public.test"
     Column |  Type   | Collation | Nullable | Default
    --------+---------+-----------+----------+---------
     x      | integer |           | not null |
    Indexes:
        "test_pkey" PRIMARY KEY, btree (x)
    
    CREATE TABLE test_with_not_null (x INT NOT NULL CONSTRAINT test_pkey_with_not_null PRIMARY KEY);
             Table "public.test_with_not_null"
     Column |  Type   | Collation | Nullable | Default
    --------+---------+-----------+----------+---------
     x      | integer |           | not null |
    Indexes:
        "test_pkey_with_not_null" PRIMARY KEY, btree (x)
    
    ALTER TABLE test DROP CONSTRAINT test_pkey;
                    Table "public.test"
     Column |  Type   | Collation | Nullable | Default
    --------+---------+-----------+----------+---------
-->     x      | integer |           | not null |
    
    ALTER TABLE test_with_not_null DROP CONSTRAINT test_pkey_with_not_null;
             Table "public.test_with_not_null"
     Column |  Type   | Collation | Nullable | Default
    --------+---------+-----------+----------+---------
-->     x      | integer |           | not null |

Here is the output in PG 17:

    CREATE TABLE test ( x INT CONSTRAINT test_pkey PRIMARY KEY );
                    Table "public.test"
     Column |  Type   | Collation | Nullable | Default
    --------+---------+-----------+----------+---------
     x      | integer |           | not null |
    Indexes:
        "test_pkey" PRIMARY KEY, btree (x)
    
    CREATE TABLE test_with_not_null (x INT NOT NULL CONSTRAINT test_pkey_with_not_null PRIMARY KEY);
             Table "public.test_with_not_null"
     Column |  Type   | Collation | Nullable | Default
    --------+---------+-----------+----------+---------
     x      | integer |           | not null |
    Indexes:
        "test_pkey_with_not_null" PRIMARY KEY, btree (x)
    
    ALTER TABLE test DROP CONSTRAINT test_pkey;
                    Table "public.test"
     Column |  Type   | Collation | Nullable | Default
    --------+---------+-----------+----------+---------
-->     x      | integer |           |          |
    
    ALTER TABLE test_with_not_null DROP CONSTRAINT test_pkey_with_not_null;
             Table "public.test_with_not_null"
     Column |  Type   | Collation | Nullable | Default
    --------+---------+-----------+----------+---------
-->     x      | integer |           | not null |

Notice that the table without a _column_ NOT NULL removes the NOT NULL
designation after removing the primary key only in PG 17.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.