Re: Fix domain fast defaults on empty tables

Heikki Linnakangas <hlinnaka(at)iki(dot)fi> writes:
> On 5 June 2026 10:48:00 EEST, Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> 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