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Re: wip: functions median and percentile

From: Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, David Fetter <david(at)fetter(dot)org>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: wip: functions median and percentile
Date: 2010-10-12 03:08:32
Message-ID: AANLkTikepCe+eSOgu_wHA1jACs94FyVrRoTN6N0FZG9M@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-rrreviewers
2010/10/12 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>:
> Hello
>
> 2010/10/11 Greg Stark <gsstark(at)mit(dot)edu>:
>> On Sun, Oct 10, 2010 at 2:16 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> It was pointed out upthread that while median isn't presently
>>> in the standard, Oracle defines it in terms of percentile_cont(0.5)
>>> which *is* in the standard.
>>
>> Uhmm, then why don't we implement that? We could provide median() as a
>> short-cut but percentile_cont() doesn't sound much harder to implement
>> than median() and more general.
>
> The problem is in interface. The original patch did it, but I removed
> it. We cannot to unsure immutability of some parameters now. Can we
> enhance a AGGREGATE to allow some mark like IMMUTABLE parameter and
> probably we should to support ANSI syntax:
>
> PERCENTILE_CONT ( expression1 )
> WITHIN GROUP ( ORDER BY expression2 [ ASC | DESC ] )
>
> This syntax allows to divide a muttable and immutable parameters.

If this is only a syntax sugar for mutable/immutable parameter, then I
guess it's time to take it serious to implement in our syntax,
although I'm not sure if it affects more execution model than
interface.

Regards,



-- 
Hitoshi Harada

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