Re: Default Value Retention After Dropping Default

On 2/24/25 03:50, Laurenz Albe wrote:
> On Mon, 2025-02-24 at 20:56 +1300, Marcelo Fernandes wrote:
>> I am experiencing an interesting behavior in PostgreSQL and would like to seek
>> some clarification.

>>
>> Can anyone explain how PostgreSQL "knows about" the default value that has just
>> been dropped and what is happened under the scenes? I am keen on a deep
>> understanding on how Postgres achieves this.
>
> The "missing value" is stored in pg_attribute.admissingval:
>
> SELECT attmissingval
> FROM pg_attribute
> WHERE attrelid = 'foo'::regclass
> AND attname = 'bar';
>
> attmissingval
> ═══════════════
> {default}
> (1 row)
>
> That value is used for all rows that don't yet physically have the column.

That answers this part of the process:

ALTER TABLE foo ADD COLUMN bar varchar(255) NOT NULL DEFAULT 'default';

I believe the OP is asking about this:

ALTER TABLE foo ALTER COLUMN bar DROP DEFAULT;

Because if after dropping the DEFAULT you do this:

INSERT INTO foo (id) SELECT generate_series(1001, 1010);

You get:

ERROR: null value in column "bar" of relation "foo" violates not-null
constraint
DETAIL: Failing row contains (1001, null).

The DEFAULT is no longer in use, but the values still exist in the
previously entered rows:

SELECT * from foo order by id desc limit 5;

id | bar
-------+---------
10000 | default
9999 | default
9998 | default
9997 | default
9996 | default

>
> Yours,
> Laurenz Albe
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com