On 22/10/13 22:56, Dimitri Fontaine wrote:
> Tom Lane <tgl@sss.pgh.pa.us> writes:
>> 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
> I just had a quick chat with a statistician friends of mine on that
> topic, and it seems that the only way to make sense of an average is if
> you know already the distribution.
>
> In our case, what I keep experiencing with tuning queries is that we
> have like 99% of them running under acceptable threshold and 1% of them
> taking more and more time.
>
> In a normal (Gaussian) distribution, there would be no query time
> farther away from the average than any other, so my experience tells me
> that the query time distribution is anything BUT normal (Gaussian).
>
>> 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?
> I don't know what multi-modal is.
>
[...]
Multi-modal is basically having more than one hump when you graph the frequencies of values.
If you gave a series of mathematical questions of varying degrees of difficulty and divers areas in mathematics to a
groupof people between the ages of 20 & 25 selected at random in New Zealand, then you would have at least 2 humps.
Onehump would be those who had little mathematical training and/or no interest and those that had had more advanced
mathematicaltraining and/or were interested in mathematics.
You would also get at least 2 humps if you plotted numbers of people under the age of 50, with the number of visits to
medicalpractioners. Basically those people with chronic illnesses with those who tend not to have extended periods of
illness- this implies 2 humps, but it may be more complicated.
Grabbing people at random and getting them to fire a rifle at targets would also be multi modal. A lot of people with
lowscores and a lessor percentage with reasonable scores. I would expect this to be quite pronounced, people with lots
ofrifle practice will tend to do significantly better.
Cheers,
Gavin
