That's not the sort of question you can answer with a couple of paragraphs in a forum. You probably need to start by reading this section of the concepts manual.http://docs.oracle.com/cd/E11882_01/ser ... .htm#i8709
Regarding the buffer cache, there are basically two options here:
- Direct Path Reads: Some operations bypass the buffer cache completely and read the information directly into the UGA.
- Everything else: An attempt is made to read the block from the cache. If it's not present, it is read from disk into the cache.