My concern is related to that. We write our own Map Reduce functions and loop thru all the records (say in millions) in the Database to select the ones we need.

My concern is related to that. We write our own Map Reduce functions and loop thru all the records (say in millions) in the Database to select the ones we need. Isn't this (looping thru all the records) costly in terms of time? I mean in RDBMS, query engine will handle this and optimize the query. In RDBMS we can even specify the indexes to make search fast but in case of NoSQL there's no schema so how does NoSQL index the data. I hardly have any experience with NoSQL so just curious. Would be nice if you can point me to some link for more detailed intro. Thanks :)