On 5/20/06, Tom Lane <tgl@xxxxxxxxxxxxx> wrote:
"Brendan Jurd" <direvus@xxxxxxxxx> writes: > I noticed a peculiarity in the default postgres aggregate functions. min()= > , > max() and avg() support interval as an input type, but stddev() and > variance() do not. > Is there a rationale behind this, or is it just something that was never > implemented? Is it sensible to calculate standard deviation on intervals? How would you handle the multiple components? I mean, you could certainly define *something*, but how sane/useful would the result be?
Strictly speaking there's nothing bad in intervals. Physically standart deviation on interval can be very useful without any doubts. I can make a lot of examples on this. Say you want to know stat parameters of semi-regular periodical process (avg distance in time between maximums of some value and stddev of this quasiperiod -- why not?). Regards, Ivan Zolotukhin
