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From: Tom Lane <[email protected]>
To: Heikki Linnakangas <[email protected]>
Cc: [email protected]
Cc: Chao Li <[email protected]>
Cc: Andrew Dunstan <[email protected]>
Cc: jian he <[email protected]>
Subject: Re: Fix domain fast defaults on empty tables
Date: Fri, 05 Jun 2026 10:08:29 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>

Heikki Linnakangas <[email protected]> writes:
> On 5 June 2026 10:48:00 EEST, Chao Li <[email protected]> wrote:
>> evantest=# create domain d_div as int check (1 / (value - 1) > 0);
>> CREATE DOMAIN
>> evantest=# create table t (a int);
>> CREATE TABLE
>> evantest=# alter table t add column b d_div default 1;
>> ERROR:  division by zero

> It seems totally reasonable to get an error in that case. '1' is not a valid value for the datatype, whether or not there are any rows in the table.

I think there's reason for concern here, which is that we do not throw
an error for the apparently equivalent case

regression=# create table t2 (a int, b d_div default 1);
CREATE TABLE

This will give you an error at INSERT, but not CREATE.  So this
is inconsistent, as well as different from the pre-v19 behavior.

Concretely, I'm pretty sure it is a hazard for pg_dump, which thinks
it can freely transform bits of CREATE operations into ALTERs.
I didn't try to make an example case, but I suspect it is now possible
to create a database that will fail dump/restore because of this
inconsistency.

			regards, tom lane

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