MongoDB does not keep cardinality statistics MongoDB tries all the possible indexes When it has found the fastest one, it remembers it MongoDB will re-try plans once in a while Indexes can be "hinted": db.cities.createIndex( { population: 1, elevation: 1 } ); db.cities.createIndex( { elevation: -1 } ); db.cities.find( { population: { $gte: 2000000 }, elevation: { $gte : 1654 } }) .sort( { elevation: -1 } ) .hint( { elevation: -1 } );