_Country Code_	_Population_	Name
AD	15853	les Escaldes
AD	20430	Andorra la Vella
...	...	...
GB	435791	Edinburgh
GB	447047	Sheffield
GB	455123	Leeds
GB	468945	Liverpool
GB	610268	Glasgow
GB	984333	Birmingham
GB	7556900	London
...	...	...

db.cities.createIndex( { country_code: 1, population: 1 } );