Re: wip: functions median and percentile

Robert Haas <robertmhaas@gmail.com> writes:
> On Sun, Oct 10, 2010 at 5:16 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. 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