Re: [PATCHES] array_accum aggregate

* Tom Lane (tgl@sss.pgh.pa.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