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Here: http://download.oracle.com/docs/html/A95907_01/diff_uni.htm#1077398
I found that on Windows Oracle is thread based, while on Unix this is process based. Why it is like that?
What’s more, there are many Oracle processes http://www.adp-gmbh.ch/ora/concepts/processes/index.html regardless the system.
Why log writer and db writer are implemented as processes… and the query execution is done using threads (windows) or processes (unix).
Oracle makes use of a SGA shared memory area to store information that is (and has to be) accessible to all sessions/transactions. For example, when a row is locked, that lock is in memory (as an attribute of the row) and all the other transactions need to see it is locked.
In windows a thread cannot access another process’s memory
As such, in Windows Oracle must be a single process with multiple threads.
On OS’s supporting the sharing of memory between processes then it is less work for Oracle to work as a multi-process architecture and leave the process management to the OS.
Oracle runs a number of background threads/processes to do work that is (or can be) asynchronous to the other processes. That way those can continue even when other processes/threads are blocked or busy.