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The Basics of an Index

Indexes help you to retrieve your data quickly- they do a “binary” search on your Oracle tables.

Choosing What Columns to Index

Choose a column, or combination of columns, which have the most unique values.  Do not choose a column or columns which have mostly the same values. If a column has mostly the same values, an index will hurt performance. First, you should know how the table is going to be used, and what data you are going after in the table. 

Source: DZone