Re: BUG #15180: Alter table add column if not exists with unique constraint will add extra duplicate

On Tue, May 1, 2018 at 11:47 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> A possible objection to this design is that right now, you can do
>
> alter table foo
> add column if not exists f2 text,
> add column if not exists f2 int;
>
> and it will skip the second ADD subcommand because by that point the
> column exists. With this design, both test subcommands would find that
> f2 doesn't exist so we'd try to do both ADD subcommands, and the second
> one would fail. That doesn't particularly bother me, because this
> command is silly.

​I'd argue its probably not that silly when you consider copy-paste errors
- in which case actually failing instead of silently ignoring the second
instance of the same name would be looked upon favorably by the user. And
since pg_dump isn't going to be affected by this the new error seems more
positive than negative on-the-whole.

As with the serial thread I'm definitely +1 for making this behave in a
less surprising manner - and Tom's proposed flow seems to me to match the
expected behavior (I agree with Stephen here, I think I got bogged down in
the sequence aspects of the previous thread when having it be this simple
is preferable).

David J.