question about sa-exim (local_scan patch) and user for spamassassin sql

Hello again,

i got another (maybe somewhat stupid) question about sa-exim.
i use exim 4.3.2 now, with local_scan patch successfully applied and 
working.
now i edited my sa-exim.conf alias spamassassin.conf and user_prefs 
files to fit my needs.

i put my spamassassin dsn-connection info for database
in .spamassassin/user_prefs in the homedirectory of exim, because spamd 
is called and running as exim
and exim has the local_scan patch, running as user exim also.

*which user is queried from database-table from dsn-connection, if 
sa-exim is applied as a local_scan patch
and running as exim? does it make use of the domainname and local_part? 
or local username? (for table column username)

*where have i exactly put the dsn-info for use with SA-exim local_scan?
is user_prefs ok? this is what exim uses for its SA preferences, e.g. 
score, the subject line etc.


thanks in advance,

yours sincerely
volker