Tom Kyte describes the 6 main reasons a index may not be used by CBO:
Superb article. Lays down the basics beautifully.
http://searchoracle.techtarget.com/tip/Why-isn-t-my-index-getting-used
A index unique scan or range scan means index is being used to improve performance of a query.
A Index skip scan or Index fast full or Index full scan scan are essentially special cases that need to be understood.. a index full scan is NOT a unique or range scan, so is prone to be a indicator of mistake if we are expecting range or unique scan by that index.
The article lays them out well.
Types of Index scans
http://www.morganslibrary.org/reference/index_scans.html
Clustering factor
http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:1032431852141
http://oracle-online-help.blogspot.com/2006/11/clustering-factor_28.html
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