Re: Fix domain fast defaults on empty tables

On Fri, Jun 5, 2026 at 10:08 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> 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.

Per the docs [1], we have two relevant forms:
ALTER [ COLUMN ] column_name SET DEFAULT expression
ADD [ COLUMN ] [ IF NOT EXISTS ] column_name data_type ... [ DEFAULT
default_expr ]

ATExecColumnDefault -> AddRelationNewConstraints does NOT evaluate the
default expression.
So ALTER COLUMN SET DEFAULT will never error out on a bad expression
because it is never evaluated at DDL time.

ALTER TABLE ADD COLUMN with a DEFAULT: pg_dump.c will never emit such a command.
You can verify this by searching for the keyword "add" in
src/bin/pg_dump/pg_dump.c, and looking at each occurrence one by one.
Looking at dumpTableSchema() confirms the same: we do not produce
command: ALTER TABLE ADD COLUMN.

See dumpTableSchema below comments related FOR LOOP part also
/*
* Dump additional per-column properties that we can't handle in the
* main CREATE TABLE command.
*/

[1] https://www.postgresql.org/docs/current/sql-altertable.html

--------------------------------------------
create or replace function dummy() returns numeric AS $$
BEGIN
RETURN 1/0;
END$$ immutable LANGUAGE plpgsql;

create table t1(a numeric default dummy());

alter table t1 add column b numeric default dummy();
ERROR: division by zero
CONTEXT: PL/pgSQL expression "1/0"
PL/pgSQL function dummy() line 3 at RETURN

As you can see, if pg_dump somehow converted the CREATE default expression into
an ALTER TABLE ADD COLUMN DEFAULT, we would already be facing this issue.

Am I missing something?

--
jian
https://www.enterprisedb.com/