Re: Ordering behavior for aggregates

On Tue, 13 Dec 2022 at 07:50, Vik Fearing <vik(at)postgresfriends(dot)org> wrote:

I am proposing something like pg_aggregate.aggordering which would be an
> enum of behaviors such as f=Forbidden, a=Allowed, r=Required. Currently
> all aggregates would have 'a' but I am thinking that a lot of them could
> be switched to 'f'. In that case, if a user supplies an ordering, an
> error is raised.
>

Although I find "r" attractive, I have two concerns about it:

1) Do we really want to require ordering? I know it's weird and partially
undefined to call something like string_agg without an ordering, but what
if in the specific application it doesn’t matter in what order the items
appear?

2) There is a backward compatibility issue here; it’s not clear to me we
could apply "r" to any existing aggregate.

Actually upon consideration, I think I have similar concerns about "f". We
don’t usually forbid "dumb" things; e.g., I can write a function which
ignores its inputs. And in some situations, "dumb" things make sense. For
example, if I’m specifying a function to use as a filter, it could be
reasonable in a particular instance to provide a function which ignores one
or more of its inputs.