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Oracle data access clarifications

I am an application developer who recently started taking on DBA activities. I am bit confused about the data access in Oracle. Normally, it is said that data access from primary memory is 10,000 times faster than disk. But how does a query determine whether the result is in data buffer cache? And if a query uses indexes it gives you the physical rowid of the row -- on the hard disk. But then how does it access from RAM?
Well, it's a misconception that disk is 10,000 times slower than memory. It actually works out to be only about 35-37 times slower BUT that's kinda beside the point. The point is, what do you need to know about when you think of physical IO (disk) vs. logical IO (memory)? Your question is not a short one to answer and in order to do it justice, I'm going to refer you to a couple of white papers that should cover everything you need to know quite nicely. All the following can be found at the Hotsos Web site in their Library. Great stuff!

1) Performance management myths and facts
2) Why a 99%+ database buffer cache hit ratio is not OK
3) Why you should focus on LIOs instead of PIOs

You'll need to register and create a userid to be able to access these documents, but it's easy to do and free.

Hope this helps!

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