What's the difference between dedicated and MTS/Shared Server in 9i, in terms of architecture and performance in a hybrid database (9 hours OLTP and 6 hours DSS)?
As you may know, in an Oracle instance using shared server, Oracle sessions do not have a dedicated server process to execute their SQL statements. Instead, each statement is passed by a dispatcher to one of the shared server processes associated with the instance. Such an architecture is useful in systems that have to support many hundreds or thousands of concurrent sessions, as it reduces the number of server processes on the Oracle host. There is slightly more CPU overhead associated with the use of shared servers, but it should be more than offset by the memory and CPU savings achieved by reducing the operating system's process management overhead.
Shared server is most effective when sessions spend a lot of their time in an idle state (as may be the case for OLTP applications). Systems with sessions that have a higher ratio of active-to-idle time (such as DSS) may not benefit as much from Shared Server. Remember, though, that it is a simple matter to specify a dedicated server process for a session even in the presence of Shared Server: Simply create a database service name that contains the term SERVER=DEDICATED, and use that service name at login time. Check the Oracle Net manual for details.