Covered Indexes
MongoDB can return data from the *index only*:
db.cities.createIndex( { country_code: 1, name: 1 } );
db.cities.find(
{ country_code: 'NL' }, *{ country_code: 1, name: 1, _id: 0 }*
).explain();
{
"cursor" : "BtreeCursor country_code_1_name_1",
"nscanned" : 256,
"nscannedObjects" : 256,
"n" : 256,
"millis" : 1,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
*"indexOnly" : true,*
...
An index needs to be used in the first place
Make sure to provide a projection
Remove %_id% explicitly from the projection