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some concepts I don't understand

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I met some terms during my study of OCA exam and the text book, Introduction to Oracle 9i SQL by Sybex, is not really helpful on explaining them. They are:
1) Index:
What are leading subset columns? How Index is created? how it is accessed? Which dictionary they are stored? Why they work differently on high and low cardinality columns? How they work if appears in Having clause?
2) Physical reads, logical reads? what do they mean?
3) init.ora file.
The only file I can find from my pc is init.ora.8252003155637.
Is this the init.ora file mentioned in the book? But, for example, if I set license_max_sessions = 3 in this file, I still can open more than 3 sessions for one user spontaneously?
Are there anyone has gone through this and want to share your knowledges?
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