From: Tom Lane <tgl@sss.pgh.pa.us>
To: "David G. Johnston" <david.g.johnston@gmail.com>
cc: "liam@morland.ca" <liam@morland.ca>,
        "pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
Subject: Re: BUG #18151: pg_upgradecluster fails when column default refers to
 column
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References: <18151-cdc2191b130172c7@postgresql.org>
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Comments: In-reply-to "David G. Johnston" <david.g.johnston@gmail.com>
	message dated "Mon, 09 Oct 2023 07:31:32 -0700"
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Date: Mon, 09 Oct 2023 11:03:37 -0400
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"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Sunday, October 8, 2023, PG Bug reporting form <noreply@postgresql.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