On 07/17/2014 11:13 PM, Fabien COELHO wrote:
However, ISTM that it is not the purpose of pgbench documentation to be a
primer about what is an exponential or gaussian distribution, so the idea
would yet be to have a relatively compact explanation, and that the
interested but clueless reader would document h..self from wikipedia or a
text book or a friend or a math teacher (who could be a friend as well:-).
Well, I think it's a balance. I agree that the pgbench documentation
shouldn't try to substitute for a text book or a math teacher, but I
also think that you shouldn't necessarily need to refer to a text book
or a math teacher in order to figure out how to use pgbench. Saying
"it's complicated, so we don't have to explain it" would be a cop out;
we need to *make* it simple. And if there's no way to do that, then
IMHO we should reject the patch in favor of some future patch that
implements something that will be easy for users to understand.
[nttcom@localhost postgresql]$ contrib/pgbench/pgbench --exponential=10
transaction type: Exponential distribution TPC-B (sort of)
scaling factor: 1
exponential threshold: 10.00000
decile percents: 63.2% 23.3% 8.6% 3.1% 1.2% 0.4% 0.2% 0.1% 0.0% 0.0%
highest/lowest percent of the range: 9.5% 0.0%
I don't have a clue what that means. None.
Maybe we could add in front of the decile/percent
"distribution of increasing account key values selected by pgbench:"
I still wouldn't know what that meant. And it misses the point
anyway: if the documentation is good, this will be unnecessary. If
the documentation is bad, a printout that tries to illustrate it by
example is not an acceptable substitute.
The decile description is quite classic when discussing statistics.
IMHO we should include a diagram for each distribution. A diagram would
be much more easy to understand than a decile or verbal explanation.
The only problem is that the build infrastructure doesn't currently
support including images in the docs. That's been discussed before, and
I think we even used to have a couple of images there a long time ago.
Now would be a good time to bite the bullet and add the support.
We got fairly close to a consensus on how to do it in this thread:
The biggest problem was choosing an editor that has a fairly stable file
format, so that we don't get huge diffs every time someone moves a line
in a diagram. One work-around for that is to use graphviz and/or gnuplot
as the source format, instead of a graphical editor.
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