Berend Tober <[EMAIL PROTECTED]> 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

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