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.
