Re: [PATCH] pgbench: new feature allowing to launch shell commands

On Tue, 22 Sep 2009, Michael Paquier wrote:

> Besides, you can also make tests without 2pc transactions, such as:
> \shell ls -ll /home/ioltas/usr/pgsql/data
> END;

I think that demonstrating the pgbench shell feature with this 2PC example
is working against your patch being even considered, much less accepted.
It's way too complicated, and the idea you're pouplarizing with it that
the script should save whatever it does in the external filesystem is
messy. You'll need a simpler one that still accomplishes something useful
that can go into the docs no matter what, the problems with prepared
transactions you're reporting in your messages are just the beginning of
issues that come from presenting this as the "hello, world" of how to use
the shell feature.

Most of the cases I can think of for situations where I'd like to call the
shell in a pgbench script require doing something useful with the result
that is returned. For example, when I'm picking a row at random, I often
want to skew which row that is, to simulate the common real-world
situation where a fraction of customers generate most of the transactions
(the Pareto principle).

What I'd consider closer to a useful implementation of this feature is
that you could call a shell script and use the first line of its output to
set one of the pgbench internal variables. The range of return codes is
way too limited for those to help here. Here's what that might look like,
assuming "skewedrand" is our script that does that given the range it
needs to operate over:

\set nbranches :scale
\set ntellers 10 * :scale
\set naccounts 100000 * :scale
\setshell aid skewedrand 1 :naccounts
SELECT abalance FROM pgbench_accounts WHERE aid = :aid;

(I'm not sure if setshell would need to interpret the output as an integer
for this to work right, there may be some int vs. string considerations
here)

If you got something like that working first before moving onto these more
complicated examples and I think you'll have an easier time of things.

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD