Can a query with an index avoid needing to read the table?

[Daniel Harman]

If I have a user object which contains Id and UserName (amongst other things). If I have a compound index on Id and username, given that the index is in memory, will the server be able to avoid even hitting the disk if I'm query for a UserName by Id?

Thanks,

Dan

[Scott Hernandez]

Yes, that is called a covered index or query. You will need to exclude
the _id field, to return only the username:

http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields#RetrievingaSubsetofFields-CoveredIndexes
http://www.mongodb.org/display/DOCS/Indexes

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[Daniel Harman]

Thanks Scott. I had no idea what to search for without knowing the correct term!