Re: gaussian distribution pgbench

Gaussian Pgbench v6 patch by Mitsumasa KONDO review & patch v7.

* The purpose of the patch is to allow a pgbench script to draw from normally
distributed or exponentially distributed integer values instead of uniformly
distributed.

This is a valuable contribution to enable pgbench to generate more realistic
loads, which is seldom uniform in practice.

* Changes

I have updated the patch (v7) based on Mitsumasa latest v6:
- some code simplifications & formula changes.
- I've added explicit looping probability computations in comments
to show the (low) looping probability of the iterative search.
- I've tried to clarify the sgml documentation.
- I've removed the 5.0 default value as it was not used anymore.
- I've renamed some variables to match the naming style around.

* Compilation

The patch applies and compiles against current head. It works as expected,
although there is few feedback from the script to show that. By looking
at the "aid" distribution in the "pgbench_history" table after a run, I
could check that the aid values are indeed skewed, depending on the parameters.

* Mathematical soundness

I've checked again the mathematical soundness for the methods involved.

After further thoughts, I'm not that sure that there is not a bias induced
by taking the second value based on "cos" when the first based on "sin"
as failed the test. So I removed the cos computation for the gaussian version,
and simplified the code accordingly. This mean that it may be a little
less efficient, but I'm more confident that there is no bias.

* Conclusion

If Mitsumasa-san is okay with the changes I have made, I would suggest
to accept this patch.

--
Fabien.