Re: Combining Aggregates

On 2/20/15 3:09 PM, Tomas Vondra wrote:
> On 20.2.2015 21:01, Peter Eisentraut wrote:
>> Is there a case where the combining function is different from the
>> transition function, other than for count?
>
> It's different in all the cases when the aggregate state is not
> identical to a single value - for example the usual avg(), sum() and
> stddev() aggregates keep state which is equal to
>
> {count(X), sum(X), sum(X*X)}
>
> The 'combine' function gets two such 'state' values, while transition
> gets 'state' + next value.

That's just because the count is hidden there in an opaque custom
transition function. If, say, we had instead an array of transition
functions {inc, plus, plussq} and we knew that plus and plussq are
associative operators, all we'd need to special case is the count case.
This would avoid a lot of repetitive code for stddev, avg, etc.

(As a bonus, you could use this knowledge to compute count, sum, avg,
and stddev in parallel using only three counters.)