C100DEV Indexing Practice Question
Which property of a Multikey Index is true when indexing an array field?
⚠ Common exam trap
Candidates often incorrectly assume that a Multikey Index creates a single index entry per array or that it cannot index arrays of objects, leading to confusion about index size.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It creates an index entry for each element in the array.
A Multikey Index is automatically created when an index is placed on a field containing an array. The index creates an entry for each element in the array, effectively mapping the index key to the parent document. This allows queries to efficiently find documents that contain specific values within their arrays. However, it also means that a single document can have multiple entries in the index, which is a key technical distinction.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A document can only have one index entry in a multikey index.
Why it's wrong here
Multikey indexes work by creating an index entry for each element within the array field. If a document's array contains five items, that document will result in five distinct entries in the index, contradicting the claim that there is only one entry per document for the indexed field.
- ✗
Multikey indexes cannot index array fields.
Why it's wrong here
The primary purpose of a multikey index is to support efficient queries on fields that hold arrays. If these indexes could not handle array fields, they would not serve their intended purpose of allowing searches on individual elements within a document's array structure in a MongoDB collection.
- ✓
It creates an index entry for each element in the array.
Why this is correct
Multikey indexes function by expanding array elements into individual index entries. This mechanism allows MongoDB to quickly query documents based on the presence of specific values inside the array, which is essential for working with data structures like tags, categories, or user roles stored as arrays in documents.
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Multikey indexes are created manually by the user.
Why it's wrong here
MongoDB automatically detects when an index is being built on an array field and designates it as a multikey index. Users do not need to specify a 'multikey' flag explicitly; the database engine manages this transformation automatically based on the data structure discovered within the collection's documents during indexing.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official MongoDB exam blueprint
This C100DEV practice question is part of Courseiva's free MongoDB certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the C100DEV exam.