MongoDB Indexing and Performance Flashcards

What is index selectivity in MongoDB performance tuning?

Answer: How effectively an index narrows down the candidate documents

Index selectivity measures how much an index reduces the number of documents that need to be examined, with higher selectivity meaning fewer documents examined.

Which command returns all indexes defined on a collection?

Answer: db.collection.getIndexes()

getIndexes() returns an array of documents describing the indexes on the collection, including their keys, names, and options.

What is the ESR rule for compound index design in MongoDB?

Answer: Equality, Sort, Range — order fields in this sequence

The ESR rule states that in a compound index, place Equality fields first, Sort fields second, and Range fields last for optimal query performance.

What happens to indexes when a MongoDB collection is dropped?

Answer: All indexes on the collection are automatically deleted

When a collection is dropped, all its indexes are automatically removed along with the collection data.

Which index option ensures no two documents can have the same value for the indexed field?

Answer: { unique: true }

Setting unique: true when creating an index causes MongoDB to reject any insert or update that would create a duplicate value for the indexed field.

What is a hashed index used for in MongoDB?

Answer: Enabling hash-based sharding by hashing field values

Hashed indexes support hash-based sharding by storing hashes of the indexed field's values, ensuring even data distribution across shards.