Re: Add min and max execute statement time in pg_stat_statement

On Mon, Oct 21, 2013 at 4:01 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Gavin Flower <GavinFlower@archidevsys.co.nz> writes:
>>>> If we're going to extend pg_stat_statements, even more than min and max
>>>> I'd like to see the standard deviation in execution time.
>
>> How about the 'median', often a lot more useful than the 'arithmetic
>> mean' (which most people call the 'average').
>
> AFAIK, median is impossible to calculate cheaply (in particular, with
> a fixed amount of workspace).  So this apparently innocent request
> is actually moving the goalposts a long way, because the space per
> query table entry is a big concern for pg_stat_statements.

Yeah, and I worry about min and max not being very usable - once they
get pushed out to extreme values, there's nothing to drag them back
toward normality except resetting the stats, and that's not something
we want to encourage people to do frequently. Of course, averages over
very long sampling intervals may not be too useful anyway, dunno.