Re: PSA: we lack TAP test coverage on NetBSD and OpenBSD

Hello Tom,

>> BTW, did you look at the question of the range of zipfian?
>
> Yep.
>
>> I confirmed here that as used in the test case, it's generating a range way 
>> smaller than the other ones: repeating the insertion snippet 1000x produces 
>> stats like this: [...]
>
>> I have no idea whether that indicates an actual bug, or just poor
>> choice of parameter in the test's call.  But the very small number
>> of distinct outputs is disheartening at least.
>
> Zipf distribution is highly skewed, somehow close to an exponential. To 
> reduce the decreasing probability the parameter must be closer to 1, eg 1.05 
> or something. However as far as the test is concerned I do not see this as a 
> significant issue. I was rather planning to submit a documentation 
> improvement to provide more precise hints about how the distribution behaves 
> depending on the parameter, and possibly reduce the parameter used in the 
> test in passing, but I see this as not very urgent.

Attached a documentation patch and a scripts to check the distribution 
(here for N = 10 & s = 2.5), the kind of thing I used when checking the 
initial patch:

   sh> psql < zipf_init.sql
   sh> pgbench -t 500000 -c 2 -M prepared -f zipf_test.sql -P 1
   -- close to 29000 tps on my laptop
   sh> psql < zipf_end.sql
  ┌────┬────────┬────────────────────┬────────────────────────┐
  │ i  │  cnt   │       ratio        │        expected        │
  ├────┼────────┼────────────────────┼────────────────────────┤
  │  1 │ 756371 │                  • │                      • │
  │  2 │ 133431 │ 5.6686302283577280 │ 5.65685424949238019521 │
  │  3 │  48661 │ 2.7420521567579787 │     2.7556759606310754 │
  │  4 │  23677 │ 2.0552012501583816 │     2.0528009571186693 │
  │  5 │  13534 │ 1.7494458401063987 │     1.7469281074217107 │
  │  6 │   8773 │ 1.5426877920893651 │     1.5774409656148784 │
  │  7 │   5709 │ 1.5366964442108951 │     1.4701680288054869 │
  │  8 │   4247 │ 1.3442429950553332 │     1.3963036312159316 │
  │  9 │   3147 │ 1.3495392437241818 │     1.3423980299088363 │
  │ 10 │   2450 │ 1.2844897959183673 │     1.3013488313450120 │
  └────┴────────┴────────────────────┴────────────────────────┘
   sh> psql < zipf_clean.sql

Given these results, I do not think that it is useful to change 
random_zipfian TAP test parameter from 2.5 to something else.

-- 
Fabien.