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Re: wip: functions median and percentile

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>, Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>, Greg Stark <gsstark(at)mit(dot)edu>, Pavel Stehule <pavel(dot)stehule(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: 2010-10-11 14:08:53
Message-ID: 22725.1286806133@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-rrreviewers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Sun, Oct 10, 2010 at 5:16 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. What I read in SQL:2008 is that
>> percentile_cont is defined for all numeric types (returning
>> approximate numeric with implementation-defined precision),
>> and for interval (returning interval), and not for any other
>> input type. So it appears to me that what we ought to support
>> is
>>    median(float8) returns float8
>>    median(interval) returns interval
>> and nothing else --- we can rely on implicit casting to convert
>> any other numeric input type to float8.

> Isn't there a possibility of a precision loss if numeric gets cast to
> float8?

So?  The standard says "implementation-defined precision".  We can
define it as giving results that are good to float8.  I find it hard to
imagine an application for median() where that's not good enough;
and what's more, the difference in comparison speed between float8 and
numeric would render median() on numeric pretty useless anyway.

			regards, tom lane

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