From: Tom Lane <tgl@sss.pgh.pa.us>
To: Heikki Linnakangas <hlinnaka@iki.fi>
cc: pgsql-hackers@lists.postgresql.org, Chao Li <li.evan.chao@gmail.com>,
        Andrew Dunstan <andrew@dunslane.net>,
        jian he <jian.universality@gmail.com>
Subject: Re: Fix domain fast defaults on empty tables
In-reply-to: <219843CF-9B49-404A-838D-88D51902B978@iki.fi>
References: <7033D663-DDB4-4B35-922C-F33DE53B1502@gmail.com>
 <219843CF-9B49-404A-838D-88D51902B978@iki.fi>
Comments: In-reply-to Heikki Linnakangas <hlinnaka@iki.fi>
	message dated "Fri, 05 Jun 2026 12:04:52 +0300"
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Date: Fri, 05 Jun 2026 10:08:29 -0400
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Heikki Linnakangas <hlinnaka@iki.fi> writes:
> On 5 June 2026 10:48:00 EEST, Chao Li <li.evan.chao@gmail.com> wrote:
>> evantest=3D# create domain d_div as int check (1 / (value - 1) > 0);
>> CREATE DOMAIN
>> evantest=3D# create table t (a int);
>> CREATE TABLE
>> evantest=3D# 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 v=
alid value for the datatype, whether or not there are any rows in the tabl=
e.

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

regression=3D# 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