How does MongoDB pick an index?

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Indexes on multi-value fields

Indexes with $or

• 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 } );