Re: [HACKERS] pgbench more operators & functions

<Oops, resent, wrong from again, I must really do something about my 1000 
mail addresses>


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 (=, <, >=).

From TPC-C 5.11 section 2.1.6, a bitwise-or operator is used to skew a 
distribution:

  | NURand (A, x, y) = (((random (0, A) | random (x, y)) + C) % (y - x + 1)) + x

And from section 5.2.5.4 of same, some time is computed based on a logarithm:

  | Tt = -log(r) * µ

> I'm also concerned that there's an opportunity cost here, in that the patch 
> establishes a precedence ordering for its new operators, which we'd have a 
> hard time changing later. That's significant because the proposed precedence 
> is different from what you'd get for similarly-named operators on the backend 
> side. I realize that it's trying to follow the C standard instead,

Oops. I just looked at the precedence from a C parser, without realizing that 
precedence there was different from postgres SQL implementation:-( This is a 
bug on my part.

> I'd be okay with the parts of this that duplicate existing backend syntax
> and semantics, but I don't especially want to invent further than that.

Okay. In the two latest versions sent, discrepancies from that were bugs, I was 
trying to conform.

Version 8 attached hopefully fixes the precedence issue raised above:

  - use precedence taken from "backend/gram.y" instead of C. I'm not sure
    that it is wise that pg has C-like operators with a different
    precedence, but this is probably much too late...

And fixes the documentation:

  - remove a non existing anymore "if" function documentation which made
    Robert assume that I had not taken the hint to remove it. I had!

  - reorder operator documentation by their pg SQL precedence.

-- 
Fabien.
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