Unable to understand the concept of Indexing

Hi,
I searched for indexing.I only understood that it is used for performance tuning.Means a indexed query will run faster ,as compared to other query that is not indexed .But i did'nt understand how it is working .It seems i have known nothing about Oracle Index. Any idea?

user12222356 wrote:
Suppose i have a small table, with two or three columns, and a few data inserted into it. Then suppose i create an index on any column, what effect will be seen here ? Correlating with books index , we are intelligent enough to understand what we are searching for .we see related topic and go to that page.Note related topic.Even we did'nt find actual topic we try to find related topics.
As for oracle tables,each columns have values, and storing info about each column will generate anther table(as i think).So two table now.So will both the tables now be queered?Such a small table can be compared with a book that has only half a page with text. You don't need any index for that book. It is faster simply to read the whole book (it is only half a page long!). In database terms this would be a full table scan (FTS).
You need an index if the book is 100 pages thick. Usually most books will have several indexes. One at the start that tells you about the differnt chapters. And a glossary at the end, where you can look up certain keywords and you also get to know on wich page those keywords had been mentions.
Lets say you have a book about animals in Africa.
So when you search for a certain keyword you have two options. You keyword would be rhino.
FTS (Full table scan)
Either read the whole book from page 1 turning to page 2 and so on until you find the keyword. And if you find it you have to read on, because maybe the keyword is mentioned again at the very last page. You need to read the whole book.
You will find some parts about white and some parts about black rhinos.
Or
Index access
You go to the glossary and search for the rhino keyword there.
Since the glossary is ordered you'll find it very fast. You will also know that you don't have to read the full glossary because it is sorted alphabetically. You then go to the page in the book (=ROWID of the table) where you find the keyword. If the glossary mentioned several pages, then you need go to all of the pages to get the full picture.
it will even more efficient if you search for a keyword like "tiger". Since a tiger does not live in Africa you would need to read the whole book just to find out that there is no "tiger"-content inside. If you use the glossary then you can fast scan the two glossary pages to see that nothing about "tiger" inside.
So an index is very efficient in telling you where to find something, but sometimes also in telling you what you will not find.
Edited by: Sven W. on Jul 13, 2010 11:12 AM

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stanwelks wrote:
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782790 wrote:
Hi Guys!
Conceptual understanding for data files.
A tablespace is having 2 datafiles.
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Moderator message - Please ask a specific question - post locked
Edited by: Rob Burbank on Oct 22, 2009 9:50 AM

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