Re: pgbench more operators & functions

Hello Tom,

> I concur that this is expanding pgbench's expression language well beyond
> what anybody has shown a need for.

As for the motivation, I'm assuming that pgbench should provide features 
necessary to implement benchmarks, so I'm adding operators that appear in 
standard benchmark specifications.

From TPC-B 2.0.0 section 5.3.5:

  | The Account_ID is generated as follows:
  | • A random number X is generated within [0,1]
  | • If X<0.85 or branches = 1, a random Account_ID is selected over all
  |   <Branch_ID> accounts.
  | • If X>=0.85 and branches > 1, a random Account_ID is selected over all
  |   non-<Branch_ID> accounts.

The above uses a condition (If), logic (or, and), comparisons (=, <, >=).