C100DBA Indexing and Performance Practice Question
Which property of a field makes it a poor candidate for an index?
⚠ Common exam trap
Candidates frequently assume indexing every field is universally beneficial, failing to recognize that low-cardinality fields degrade performance and waste resources.
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
✓
The field has very low cardinality.
Low cardinality fields, such as those with only two or three unique values, provide little benefit when indexed. MongoDB's query optimizer often skips these indexes because a collection scan is usually faster than traversing the B-tree for a large percentage of the dataset. Identifying high-cardinality fields is essential for building effective indexes that genuinely speed up data retrieval rather than just adding overhead to write operations.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The field is frequently used in filter criteria.
Why it's wrong here
Fields used in filter criteria are ideal candidates for indexing. By indexing these fields, MongoDB can quickly locate specific documents without scanning the entire collection, which directly improves query performance and reduces the overall load on the database engine during data retrieval tasks.
- ✗
The field contains unique identifiers like UUIDs.
Why it's wrong here
Fields with unique values are high-cardinality and are perfect candidates for indexing. An index on a unique field allows the database to locate a single document in logarithmic time, which is highly efficient for lookup-heavy workloads and essential for enforcing uniqueness constraints in the schema.
- ✓
The field has very low cardinality.
Why this is correct
Low cardinality means the field has very few unique values across a large number of documents. When you search for one of these values, the index may return a huge portion of the collection, making a full collection scan more performant than using the index itself.
- ✗
The field is a numeric timestamp.
Why it's wrong here
Numeric timestamps are typically high-cardinality and are excellent candidates for indexing. They are frequently used for time-series range queries. Indexing a timestamp allows for efficient retrieval of recent data and helps optimize operations like sorting by time, which is a common requirement in many applications.
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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 C100DBA 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 C100DBA exam.