Re: Add min and max execute statement time in pg_stat_statement

On 10/21/2013 07:29 PM, Tom Lane wrote:
> Andrew Dunstan <andrew@dunslane.net> writes:
>> This is why I suggested the standard deviation, and why I find it would
>> be more useful than just min and max. A couple of outliers will set the
>> min and max to possibly extreme values but hardly perturb the standard
>> deviation over a large number of observations.
> Hm.  It's been a long time since college statistics, but doesn't the
> entire concept of standard deviation depend on the assumption that the
> underlying distribution is more-or-less normal (Gaussian)?  Is there a
> good reason to suppose that query runtime is Gaussian?  (I'd bet not;
> in particular, multimodal behavior seems very likely due to things like
> plan changes.)  If not, how much does that affect the usefulness of
> a standard-deviation calculation?


IANA statistician, but the article at 
<https://en.wikipedia.org/wiki/Standard_deviation> appears to have a 
diagram with one sample that's multi-modal.

cheers

andrew