I am developing an application in JBoss with Firebird as the
datastore.

I am toying (at this initial stage) between numeric primary keys and
string based primary keys.

The latter because I could used a uuid generator within my EJB
(http://www.activescript.co.uk/jguid.html) to get a (almost) unique
key (in both space and time).

However, after a lot of "google" searches I have yet to understand
conclusively whether a numeric PK would be more "efficient" to index
as opposed to a varchar(36) key (particularly when the DB has between
100,000 to a millon records to sift through).

I would appreciate your thoughts on this matter (and also if the
argument would apply to most RDBMS's).

Cheers
-raj