Re: [PATCH] Fix ALTER SYSTEM empty string bug for GUC_LIST_QUOTE parameters

Jim Jones <jim(dot)jones(at)uni-muenster(dot)de> writes:
> On 04.09.25 23:52, Tom Lane wrote:
>> I'm not entirely sure if this is the way to go, or if we want to
>> adopt some other solution that doesn't involve forbidding empty
>> list elements. I suspect that anything else we come up with would
>> be less intuitive than letting SET list_var = '' do the job;
>> but maybe I just lack imagination today.

> The ambiguity between an empty list and an empty element has always
> existed in list-valued GUCs. This patch resolves the issue by
> disallowing empty elements, thereby making '' an unambiguous
> representation of an empty list. Personally, I find SET var TO NULL (or
> perhaps a keyword like EMPTY or NONE) a more palatable syntax for
> expressing empty lists in this case. However, I’m not sure the
> additional complexity and compatibility implications would justify such
> a change.

Since you expressed interest, I made a draft patch that does it like
that. Unsurprisingly, it has to touch mostly the same places that
the v3 patch did, plus the grammar. Still ends up a bit shorter
though.

I remain unsure which way I like better. The NULL approach has the
advantage of not foreclosing use of empty-string list elements, which
we might want someday even if there's no obvious value today. (And
for the same reason, it's less of a behavioral change.) But it still
feels a bit less intuitive to me. It might flow better with some
other keyword --- but we have to use a fully-reserved keyword, and we
are surely not going to make a new one of those just for this purpose,
and NULL is the only existing one that's even slightly on-point.

regards, tom lane