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.

• ✗

  The index can support a sort on price descending even if the query does not include any condition on category, because the index stores price in descending order when traversed in reverse.

  Why it's wrong here

  This is incorrect because the index is not globally sorted by price. It is sorted by category first, then price. Without an equality condition on category, the index cannot provide a global sort on price descending. Scanning in reverse would yield a global order by category descending and price descending within each category, which is not the same as a global price descending sort. An in-memory sort would still be required.

• ✓

  The index can support a sort on price descending if the query includes an equality condition on category, by scanning the index in reverse.

  Why this is correct

  This is true because MongoDB can traverse an index in either direction. With an equality condition on category, the index entries for a given category are ordered by price ascending. To satisfy a sort on price descending, MongoDB can scan those entries in reverse order, avoiding an in-memory sort. This is a standard optimization.

• ✓

  The index can support a sort on price ascending if the query includes an equality condition on category.

  Why this is correct

  This is true because when there is an equality predicate on category, the index's remaining field price is effectively sorted in ascending order (since the index is { category: 1, price: 1 }). The query can use the index to return results sorted by price ascending without an in-memory sort, as long as the equality condition on category is present.

• ✗

  The index can support a sort on price ascending if the query includes a range condition on category, such as { category: { $gt: 'A' } }.

  Why it's wrong here

  This is false because a range condition on category does not fix the category to a single value. The index entries will be ordered by category first, and within each category by price. For a range of categories, the overall order is not by price globally. Therefore, the index cannot provide a sort on price ascending without an in-memory sort, even though it can be used for filtering on the category range.

• ✗

  The index can support a sort on price ascending even if the query does not include any condition on category.

  Why it's wrong here

  This is false because without a condition on category, the index is ordered first by category and then by price within each category. A global sort on price would require merging results from multiple categories, which the index cannot provide directly. MongoDB would need an in-memory sort to order all documents by price across all categories, so the index alone cannot support the sort.