Multikey indexes for arrays in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a multikey index in MongoDB?

A multikey index is an index that MongoDB creates on fields that hold arrays. It indexes each element of the array separately, allowing efficient queries on array contents.

beginner

How does MongoDB handle indexing when a document field contains an array?

MongoDB creates a multikey index by indexing each element of the array as if it were a separate value, enabling queries to match any element inside the array.

intermediate

Can a multikey index be created on multiple array fields in the same document?

No, MongoDB does not support multikey indexes on multiple array fields in the same document because it would create a combinatorial explosion of index entries.

beginner

What is a practical example of a query that benefits from a multikey index?

If a collection has documents with a field 'tags' that is an array, a query like { tags: 'mongodb' } can quickly find documents containing 'mongodb' in the tags array using a multikey index.

intermediate

What happens if you create a multikey index on a field that sometimes contains arrays and sometimes single values?

MongoDB treats single values as arrays with one element and indexes them the same way, so the multikey index works seamlessly for both cases.

What does a multikey index in MongoDB index?

A Only the first element of an array

B The entire array as a single value

C Each element of an array field separately

D Only non-array fields

A multikey index indexes each element of an array field separately to allow efficient queries on any array element.

Can MongoDB create a multikey index on two different array fields in the same document?

A Yes, always

B No, it is not supported

C Only if arrays have the same length

D Only in MongoDB versions before 4.0

MongoDB does not support multikey indexes on multiple array fields in the same document to avoid combinatorial index explosion.

If a field contains a single value instead of an array, how does a multikey index treat it?

A Indexes it as a single-element array

B Ignores the value

C Throws an error

D Indexes only if it is a string

MongoDB treats single values as arrays with one element when creating a multikey index.

Which query benefits most from a multikey index on an array field 'tags'?

A { "tags": { "$size": 3 } }

B { "tags": ["mongodb", "database"] }

C { "tags": { "$exists": true } }

D { "tags": "mongodb" }

A query searching for a specific element in the array, like { tags: 'mongodb' }, uses the multikey index efficiently.

What is a limitation of multikey indexes in MongoDB?

A They cannot index multiple array fields in the same document

B They cannot be used on array fields

C They cannot index nested documents

D They slow down all queries

Multikey indexes cannot be created on multiple array fields in the same document due to index complexity.

Explain what a multikey index is and how it works with array fields in MongoDB.

Think about how MongoDB handles arrays when creating indexes.

Describe a scenario where a multikey index would improve query performance and a limitation to be aware of.

Consider both benefits and restrictions of multikey indexes.