Re: Add min and max execute statement time in pg_stat_statement

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?
        regards, tom lane