Can someone tell me how indexes work ?
Does index stores some kind of number which select statement then accesses to make query faster ?
Is it stored in memory ? If so what happens when some one does DML on that table. Now you try to do select, how does index knows the new data ?

I read many documents on this but this is not explained any where. It is just said that good indexes improves performance, which I think everyone knows. How ??

Thanks
Sonali