Horton

Re: wip: functions median and percentile

Pavel Stehule <pavel.stehule@gmail.com>

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From: Pavel Stehule <pavel.stehule@gmail.com>
To: Greg Stark <gsstark@mit.edu>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Dean Rasheed <dean.a.rasheed@gmail.com>, Hitoshi Harada <umi.tanuki@gmail.com>, Robert Haas <robertmhaas@gmail.com>, David Fetter <david@fetter.org>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2010-10-11T18:46:47Z
Lists: pgsql-hackers 
Hello

2010/10/11 Greg Stark <gsstark@mit.edu>:
> On Sun, Oct 10, 2010 at 2:16 PM, Tom Lane <tgl@sss.pgh.pa.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.

Regards

Pavel Stehule
>
> --
> greg
>