Re: Aggregate ORDER BY patch

2009/11/14 Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>:
>>>>>> "Heikki" == Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
>
>  >> No artificial restrictions are imposed on what syntactical
>  >> combinations are allowed. However, ORDER BY is not allowed with
>  >> aggregates used as window functions (as per the existing
>  >> restriction on DISTINCT).
>
>  Heikki> How is this different from window functions?
>
> Window functions return a row for each row of input, aggregates don't.
>
> The reason I didn't tackle the case of aggregate functions used as
> window functions is that the spec allows constructs like this:
>
> array_agg(a order by b) over (order by c)
>
> which can't be represented using the aggregate-as-window-function
> mechanism as it currently stands, since you'd have to re-sort the
> window each time.
>
Now I'm about to send my patch to introduce more frame types,
aggregate cache mechanism in window functions may be broken sometimes,
and it is *possible* to put order-by clause in argument list if we
prepare tuplesort as in nodeAgg. But I don't see useful cases and it
seems so hard task that I'm not sold.

Regards,

--
Hitoshi Harada