Server processes are those that perform work on behalf of a client session. They are the processes that ultimately receive and act on the SQL statements our applications send to the database.
In Chapter 2, we briefly touched on the two main connection types to Oracle, namely, the following:
•\ Dedicated server: There is a dedicated process on the server for your connection. There is a one-to-one mapping between a connection to the database and a server process or thread.
•\ Shared server: Many sessions share a pool of server processes spawned and managed by the Oracle instance. Your connection is to a database dispatcher, not to a dedicated server process created just for your connection.
Note It is important to understand the difference between a connection and a session in Oracle terminology.
A connection is just a physical path between a client process and an Oracle instance (e.g., a network connection between you and the instance).
A session, on the other hand, is a logical entity in the database, where a client process can execute SQL and so on.
Many independent sessions can be associated with a single connection, and these sessions can even exist independently of a connection. We will discuss this further shortly.
Both dedicated and shared server processes have the same job: they process all of the SQL you give to them.
When you submit a SELECT * FROM EMP query to the database, an Oracle dedicated/shared server process parses the query and places it into the shared pool (or finds it in the shared pool already, hopefully).
This process comes up with the query plan, if necessary, and executes the query plan, perhaps finding the necessary data in the buffer cache or reading the data from disk into the buffer cache.
These server processes are the workhorse processes. Often, you will find these processes to be the highest consumers of CPU time on your system, as they are the ones that do your sorting, your summing, your joining—pretty much everything.