On 22/10/13 09:01, Tom Lane 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.
>
> regards, tom lane
Yeah, obvious - in retrospect! :-)
One way it could be done, but even this would consume far too much
storage and processing power (hence totally impractical), would be to
'simply' store a counter for each value found and increment it for each
occurence...
Cheers,
Gavin