PLPGSQL 'SET SESSION ROLE' problems ..

Hi,

Is it possible to change the current role to another arbitrary role
using a PLPGSQL function?

In essence I've a function "authorise" created by the db superuser with
'SECURITY DEFINER' specificied.

However from within plpgsql the 'superuser' attribute isn't honored when
trying to change roles (ie: the non 'superuser' rules for role change
must be honoured).

Postgresql version 8.2.3

Is this a bug? .. If not, how do I achieve an 'adhoc' change of role
from within the "rules system"?

tnx,

   -greg

--
Dr. Greg Wickham                  Program Manager, e-Research
Phone: +61 2 6222 3552                                 AARNet
Mob: +61 4 0785 4566              CSIRO, Building 9, Banks St
Email: greg.wickham@aarnet.edu.au         Yarralumla ACT 2600