Re: BUG #15579: Adding a column with default from configuration parameter fails on 11.1

On 2019-01-07 16:01:43 -0500, Tom Lane wrote:
> Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
> > Stable expressions are quite ok for fast defaults. The expression is
> > evaluated once when the ALTER TABLE is done and the result (not the
> > expression) is stored in the catalog. The reason we check for volatile
> > expressions is precisely because we don't want all the existing rows to
> > get a single value in that case. This was discussed during the Postgres
> > 11 development cycle.
>
> Hmm.
>
> The issue here is that if the table is empty, the old behavior evaluated
> the expression zero times during ALTER TABLE. Now we evaluate it once,
> and if that throws an error, that's a user-visible behavior change.
>
> Perhaps it's okay to decide that that's an acceptable behavioral change,
> but it makes this feature less transparent than it was supposed to be.

It doesn't seem too hard to scan far enough to see whether there's a
single non-dead row. So we could fix this, if we wanted.

But I'm disinclined to think that it's worth doing so - and there could
be drawbacks, e.gg tables that are all-dead. Given the IMO quite minor
behaviour change, I'm thus disinclined to "fix" this..

Greetings,

Andres Freund