|From:||Marc Mamin <M(dot)Mamin(at)intershop(dot)de>|
|To:||"'KONDO Mitsumasa'" <kondo(dot)mitsumasa(at)lab(dot)ntt(dot)co(dot)jp>, Stephen Frost <sfrost(at)snowman(dot)net>, Dimitri Fontaine <dimitri(at)2ndQuadrant(dot)fr>|
|Cc:||Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andrew Dunstan <andrew(at)dunslane(dot)net>, "Peter Geoghegan" <pg(at)heroku(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>|
|Subject:||Re: Add min and max execute statement time in pg_stat_statement|
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>> Oscillating plan changes may fit multimodal but I don't feel that's
>> typical. My experience has been it's either an extremely rare plan
>> difference or it's a shift from one plan to another over time.
>After all, all of avg, min, max and stdev are only numerical value for predicting model. There aren't the robustness and strictness such as Write Ahead Logging. It resembles a weather forecast. They are still better than nothing.
>It is needed a human judgment to finally suppose a cause from the numerical values. By the way, we can guess probability of the value from stdev.
>Therefore we can guess easily even if there is an extreme value in min/max whether it is normal or not.
>>> What I've been gathering from my quick chat this morning is that
>>> either you know how to characterize the distribution and then the min
>>> max and average are useful on their own, or you need to keep track of
>>> an histogram where all the bins are of the same size to be able to
>>> learn what the distribution actually is.
We have an in house reporting application doing a lot of response times graphing.
Our experience has shown that in many cases of interest (the one you want to dig in)
a logarithmic scale for histogram bins result in a better visualization.
attached an example from a problematic postgres query...
my 2 pences,
|Next Message||Martijn van Oosterhout||2013-10-23 21:46:01||Re: Add min and max execute statement time in pg_stat_statement|
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