Re: gaussian distribution pgbench

Gaussian Pgbench v8 patch by Mitsumasa KONDO review & patch v9.

* 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.

* Very minor change

I have updated the patch (v9) based on Mitsumasa latest v8:
- remove one spurious space in the help message.

* Compilation

The patch applies cleanly and compiles against current head.

* Check

I have checked that the aid values are skewed depending on the
parameters by looking at the "aid" distribution in the "pgbench_history"
table after a run.

* Mathematical soundness

I've checked the mathematical soundness of the methods involved.

I'm fine with casting doubles to integers for having the expected
distribution on integers.

Although there is a retry loop for finding a suitable, the looping
probability is low thanks to the minimum threshold parameter required.

* Conclusion

I suggest to apply this patch which provide a useful and more realistic
testing capability to pgbench.

--
Fabien.