Re: [HACKERS] array_accum aggregate

* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> That's not really the flavor of solution I'd like to have. Ideally,
> it'd actually *work* to write
>
> my_ffunc(my_sfunc(my_sfunc(null, 1), 2))
>
> and get the same result as aggregating over the values 1 and 2. The
> trick is to make sure that my_sfunc and my_ffunc can only be used
> together. Maybe we do need a type for each such aggregate ...

In general I like this idea but there are some complications, the main
one being where would the memory be allocated? I guess we could just
always use the QueryContext. The other issue is, in the above scenario
is it acceptable to modify the result of my_sfunc(null, 1) in the ,2
call? Normally it's unacceptable to modify your input, aggregates get a
special exception when called as an aggregate, but in this case you'd
have to copy the entire custom structure underneath, I'd think.

As for a type for each such aggregate, that seems reasonable to me,
honestly. I'd like for the type creation to be reasonably simple
though, if possible. ie: could we just provide NULLs for the
input/output functions instead of having to implement functions that
just return an error? Or perhaps have a polymorphic function which can
be reused that just returns an error. Additionally, we'd have to be
able to mark the types as being polymorhpic along the same lines as
anyelement/anyarray. Hopefully that's already easy, but if not it'd be
nice if it could be made easy...

Thanks,

Stephen