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Message-ID: <ccb92885-a620-4760-860e-91286038e66f@aklaver.com>
Date: Mon, 24 Feb 2025 08:37:06 -0800
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: Default Value Retention After Dropping Default
To: Laurenz Albe <laurenz.albe@cybertec.at>,
 Marcelo Fernandes <marcefern7@gmail.com>, pgsql-general@lists.postgresql.org
References: <CAM2F1VNZNeruO2N+_-Fq3JJ7bto35kOzomoN5A2dmstAWLi5tQ@mail.gmail.com>
 <efdbe967d39597898f1730803e34798cf5016562.camel@cybertec.at>
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From: Adrian Klaver <adrian.klaver@aklaver.com>
In-Reply-To: <efdbe967d39597898f1730803e34798cf5016562.camel@cybertec.at>
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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.klaver@aklaver.com