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Re: wip: functions median and percentile

From: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
To: David Fetter <david(at)fetter(dot)org>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Stark <gsstark(at)mit(dot)edu>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: wip: functions median and percentile
Date: 2010-08-19 17:38:14
Message-ID: AANLkTinoEu0JZuwWcrCP4iyOXUjPH-wxTcuPOJUGvHKR@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-rrreviewers
2010/8/19 David Fetter <david(at)fetter(dot)org>:
> On Thu, Aug 19, 2010 at 12:49:45PM -0400, Robert Haas wrote:
>> On Thu, Aug 19, 2010 at 11:33 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> > Greg Stark <gsstark(at)mit(dot)edu> writes:
>> >> On Thu, Aug 19, 2010 at 11:59 AM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>> >>> I am sending a prototype implementation of functions median and
>> >>> percentile. This implementation is very simple and I moved it to
>> >>> contrib for this moment - it is more easy maintainable. Later I'll
>> >>> move it to core.
>> >
>> >> So if the entire result set fits in memory it would be nice to use the
>> >> O(n) Quickselect algorithm -- which would only be a small change to
>> >> the existing Quicksort code -- instead of sorting the entire set.
>> >
>> > That seems like rather a lot of added infrastructure for functions whose
>> > popularity is at best unknown.  I think we should KISS for the first
>> > implementation.
>>
>> +1.  I think the functions are useful, but the perfect is the enemy of the good.
>
> Percentile is already there as NTILE, a windowing function.  Median

I don't think it. The purpose is same, but usage is different -
aggregate functions can be used as window func, but window functions
cannot be used as aggregates.


> may be useful, but we pretty much can't just call it "median."
> Instead, we need to call it something like "left_median" or
> "arithmetic_median."

I put some time to deep searching in this area - and I talked with my
kolegues mathematican and everywhere use a just median. Some longer or
different name isn't barrier for me, but people can be confused.

Regards
Pavel

>
> Cheers,
> David.
> --
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