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Oracle Tips by Burleson 

Indexes

So, you go to the library to find a book to read.  It is summer, and you have time on your hands and are not sure what you want.  You probably browse the shelves looking for something that might interest you.  When you’re not sure what you want, this is a pretty easy access method, browsing the shelves.  But a database never browses for a row. It is always looking for a particular row or rows.   

So, let’s go back to the library and look for a particular book.  In this instance, you would not go and browse the shelves; you would head for the card catalog.  It could take you hours to find a book by searching the shelves, while it will take only minutes to look up the book and go directly to the shelf where the book is located.  This is because all of the books in the library are numbered using the Dewey Decimal system.  Sometimes, you are not sure which book you want but know the general subject.  You can look up the subject in the card catalog and then go to the shelves and look at a range of books.   

Oracle uses basically the same system, except that the card catalog is an index and the book number becomes the rowid.  When a query asks for a specific row of data, an index allows the database to look up the rowid of that specific row and directly retrieve it.  Otherwise, the database must start at the beginning of the table and check each row to find the data it wants.


The above book excerpt is from:

Easy Oracle SQL

Get Started Fast writing SQL Reports with SQL*Plus

ISBN 0-9727513-7-8

John Garmany 

http://www.rampant-books.com/book_2005_1_easy_sql.htm

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