Re: wip: functions median and percentile

2010/10/1 Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>:
> 2010/10/2 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
>> Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com> writes:
>>> 2010/10/2 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
>>>> The implementation I would've expected to see is to do the sort and then
>>>> have two code paths for retrieving the median, depending on whether the
>>>> sort result is all in memory or not.
>>
>>> Hm? The problem we encountered in the middle of the patch is there is
>>> no chance to call tuplesort_end if median is called in moving frame
>>> window aggregate because final function is called multiple times
>>> during moving.
>>
>> Well, if you haven't got a solution for that, then this patch isn't
>> ready for prime time.
>>
>> It's entirely possible that median as a window function is intractable.
>> I'd rather have it throwing error than offer an implementation that will
>> fall over as soon as the window gets large.
>
> Well, that sounds like the conclusion. It is a shame, but we have to
> throw an error from median() in the window aggregate, if Pavel does
> not have any better solution. And as an aggregate function only, the
> patch is ready if the window-related parts are removed.
>

I am sorry - I don't have a better solution. Classic algorithm isn't
well for window aggregate - it needs a sort after any append a new
item. Maybe we can use a separate functionality based on estimated
values for a windows. I read some articles about it. But this is work
on longer time - all articles about this topic are experimental. More
I am not mathematician - so I am not able to review these methods.
Today or tomorrow I'll send a updated patch without support a window
aggregates.

Regards

Pavel Stehule

> Regards,
>
>
> --
> Hitoshi Harada
>