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.

• ✗

  MongoDB will use the partial index to find documents with price less than 50.

  Why it's wrong here

  MongoDB will not use the partial index for this query because the query predicate `{ price: { $lt: 50 } }` does not match the partial filter expression `{ price: { $gt: 100 } }`. The partial index only contains entries for documents where `price` is greater than 100. Since the query is looking for prices less than 50, those documents are not in the index. Therefore, the index cannot be used to satisfy the query, and MongoDB will not use it.

• ✗

  MongoDB will use the partial index but will filter out documents that do not match the query predicate.

  Why it's wrong here

  This is incorrect because the partial index does not contain any documents with `price < 50`. The index only has entries for `price > 100`. Therefore, if MongoDB tried to use the index, it would find no matching entries for the query. MongoDB's query planner recognizes that the partial index is not useful for this query and will not use it. It does not use the index and then filter; it simply avoids the index entirely.

• ✓

  MongoDB will perform a collection scan to find documents with price less than 50.

  Why this is correct

  MongoDB will perform a collection scan because the partial index is not applicable to the query. The partial index only includes documents where `price > 100`, and the query is for `price < 50`. Since the query predicate does not overlap with the partial filter expression, the index cannot be used. MongoDB will fall back to a collection scan to find the matching documents, examining every document in the collection. This is less efficient but necessary because no suitable index exists for this query.

• ✗

  MongoDB will return an error because the query cannot be satisfied by the partial index.

  Why it's wrong here

  MongoDB does not return an error when a partial index cannot be used for a query. Instead, it will simply not use the index and will choose an alternative plan, such as a collection scan. The query will still execute and return results, albeit potentially slower. Errors are not thrown for index selection; the query planner evaluates available indexes and picks the best one, or falls back to a collection scan if none are suitable.