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
integervalues 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
thresholdparameter required.
 

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

-- 
Fabien.