Re: BUG #18151: pg_upgradecluster fails when column default refers to column

"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Sunday, October 8, 2023, PG Bug reporting form <noreply(at)postgresql(dot)org>
> wrote:
>> The reason is the the default value for column "event_id" of table "event"
>> is "event_id_nextval()". That function is:
>> SELECT COALESCE(MAX(event_id) + 1, 1) FROM event;

> You are expressly forbidden to have the default value expression of a
> column reference a subquery. You must use a trigger.

Well, you're not allowed to do it directly:

regression=# create table foo (f1 int default ((select max(f1) from foo)));
ERROR: cannot use subquery in DEFAULT expression
LINE 1: create table foo (f1 int default ((select max(f1) from foo))...
^

but that's just a minor implementation restriction. Doing it through
a function is fine. (Whether it's a good idea is another question.)
Unlike, say, CHECK constraints, there's not any expectation that
a default expression be immutable or avoid dependence on database
state --- if there was, "default nextval(...)" would be problematic.

In any case, dump/restore ought not fail like this.

regards, tom lane