Berend Tober <btober(at)seaworthysys(dot)com> writes:
> I'm stuck on not knowing how to define a
> aggregate that takes more that one variable as its argument,
That's because there isn't any way to do that. It's on the TODO list
I believe. In the meantime, you could possibly kluge it up by
defining a composite type to be the aggregate's argument, and calling
it like
SELECT covariance(row(...)) FROM ...
regards, tom lane