random char or text variable in pgbench

Hi all,

I am currently playing with the nice pgbench tool.
I would like to build a benchmark using pgbench with customized scenarios,
in order to get something quite representative of a real workload.
I have designed a few tables, with a simple script to populate them, and
defined 3 scenarios representing typical transactions.

But I have the following issue. Some tables have CHAR (or TEXT or VARCHAR)
columns that belong to their primary key and I want to include into
pgbench scenarios statements with conditions on these CHAR columns, using
some random values generated by pgbench.
As pgbench \set, \setrandom or \setshell meta-commands only manage integer
variables, I tried to use SQL conditions like:
... where my_column = to_char(:var::integer,'00009FM') ...
with var previously defined by:
\setrandom var 1 :maxvar

Having previously loaded my_column with digits strings, I get the right
result. But ... this condition cannot use any index (tested on 9.2beta2).
As a result, I get looooooong index or table scans, which is of course not
acceptable in my benchmark as it is not representative of the real data
access path :-((

Does someone has a trick to manage random char or text variable in pgbench
?

Thanks by advance for any help.
Best regards.
Philippe Beaudoin.