C100DEV Indexing Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  Creating a unique index on email will prevent insertion of documents with duplicate email values, but will allow multiple documents with missing email field only if the index is sparse or partial.

  Why this is correct

  A unique index enforces uniqueness on the indexed field, but by default it treats missing fields as null and allows only one document without the field. To allow multiple documents missing the email field while still enforcing uniqueness when present, you must create the index with the sparse option or a partial filter expression. This is a common requirement for optional unique fields.

• ✗

  A compound index on { country: 1, city: 1 } can support queries that filter on city alone, because MongoDB can use the index prefix from the right side.

  Why it's wrong here

  MongoDB compound indexes support queries that use a prefix of the index fields, starting from the leftmost field. An index on { country: 1, city: 1 } can support queries on country alone or on both country and city, but not on city alone. To support city-only queries, you would need a separate index with city as the leading field. This statement incorrectly describes index prefix usage.

• ✓

  A compound index on { country: 1, city: 1 } can be used for a query that filters on both country and city, and also for a query that filters on country alone.

  Why this is correct

  A compound index supports queries that use a prefix of its fields. With { country: 1, city: 1 }, the index can be used for queries filtering on country, or on both country and city. The leading field country is the prefix, so country-only queries can use the index efficiently. Queries on city alone cannot use it as a prefix, but that is not part of this statement.

• ✗

  Creating a unique index on username will automatically create a unique index on email if both are specified in the same createIndex command using a compound key pattern.

  Why it's wrong here

  A compound unique index on { username: 1, email: 1 } enforces uniqueness across the combination of username and email, not on each field individually. Two documents could have the same username with different emails, or the same email with different usernames, and still satisfy the compound uniqueness. Separate unique indexes are required to enforce uniqueness independently on each field.

• ✗

  Unique indexes in MongoDB enforce uniqueness across all fields in the document, not just the indexed field, to prevent duplicate documents.

  Why it's wrong here

  Unique indexes enforce uniqueness only on the specified key pattern. They do not compare entire documents. For example, a unique index on email only prevents two documents from having the same email value; other fields can be identical. To prevent duplicate documents entirely, you would need a unique index on all fields or a different mechanism, which is not how unique indexes work.