Re: Add min and max execute statement time in pg_stat_statement

Andrew Dunstan schrieb am 2015-01-20:

> On 01/20/2015 01:26 PM, Arne Scheffer wrote:
> >Interesting patch.
> >I did a quick review looking only into the patch file.

> >The "sum of variances" variable contains
> >the "sum of squared differences" instead, I think.

> Umm, no. It's not.

Umm, yes, i think, it is ;-)

> e->counters.sum_var_time +=
> (total_time - old_mean) * (total_time - e->counters.mean_time);
> This is not a square that's being added.

That's correct.
Nevertheless it's the difference between the computed sum of squared
differences
and the preceeding one, added in every step.

> old_mean is not the same as
> e->counters.mean_time.

> Since the variance is this value divided by (n - 1), AIUI, I think
> "sum
> of variances" isn't a bad description. I'm open to alternative
> suggestions.

> >And a very minor aspect:
> >The term "standard deviation" in your code stands for
> >(corrected) sample standard deviation, I think,
> >because you devide by n-1 instead of n to keep the
> >estimator unbiased.
> >How about mentioning the prefix "sample"
> >to indicate this beiing the estimator?

> I don't understand. I'm following pretty exactly the calculations
> stated
> at <http://www.johndcook.com/blog/standard_deviation/>

(There is nothing bad about that calculations, Welford's algorithm
is simply sequently adding the differences mentioned above.)

VlG-Arne

> I'm not a statistician. Perhaps others who are more literate in
> statistics can comment on this paragraph.

> >And I'm sure I'm missing C specifics (again)
> >(or it's the reduced patch file scope),
> >but you introduce sqrtd, but sqrt is called?

> Good catch. Will fix.

> cheers

> andrew