Re: I probably don't understand aggregates.

On 12.06.2014 00:09, Tom Lane wrote:
> That is certainly not going to do what you presumably want. The outer
> query doesn't think that the wrapper function is an aggregate; it's
> just a plain scalar function that will be called separately on each
> row of the table. Meanwhile, the best way to think about what's
> happening inside the wrapper is that M := myAggrFunction(x); is
> equivalent to SELECT myAggrFunction(x) INTO M FROM
> anonymous-one-row-table; or if you're an Oracle refugee, SELECT
> myAggrFunction(x) INTO M FROM dual; So the aggregate is evaluated over
> just one row, and I suppose the zero-divide error happens because your
> final function is failing to cope with that case. regards, tom lane

On 12.06.2014 00:08, David G Johnston wrote:
> Because myWrapperFunction is not an aggregate you end up calling
> myAggr three times, once each with for the values 0, 0, 1. The way
> myAggr is written if the final state of the function is 0 a division
> by zero will occur. My privately noted confusion about the lack of a
> GROUP BY still applies - I just forgot about the implicit GROUP BY
> when all output columns are defined using aggregates. Try executing:
> SELECT myAggr(x), x FROM test GROUP BY x; This is basically what you
> are doing when you put the aggregate into the wrapper. David J. --
> View this message in context:
> http://postgresql.1045698.n5.nabble.com/I-probably-don-t-understand-aggregates-tp5806879p5806896.html
> Sent from the PostgreSQL - novice mailing list archive at Nabble.com.

Thank you both, that helped a lot.

I'm doing a project for my informatics minor, where I have to use
aggregates to get some matrix multiplication and inversion done inside
pgsql.

But how can I get my wrapper function to evaluate the aggregate on the
whole table and work with the result afterwards?

Or do I need to change the aggregate?

Markus