Can anyone give a clear,short and simple explanation about database indexes?

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Do you have a particular RDBMS in mind?

@diafol thanks..yes taking RDBMS in mind Indexes how plays role in databases?

@gusano79 i m not asking about first paragrah of wiki that i already can see but i need a clear explanation that can help in understanding indexes well...i am bit of confused in keys and indexes.

It's like using an index at the back of a book to get the page (address) of a particular topic. Instead of scanning the book looking for the topic you just get the address from the index and go directly there. You can add an index for any field (or combination of fields) but with every index you add there is a penalty in that it requires extra space for the index and extra processing time to maintain the index.

Thanks for the clear explanation Jim.