MongoDB Updated C100DBA Exam Questions and Answers by ciara

A hidden index remains physically present and continues to be maintained as documents are inserted, updated, or deleted, but the query planner does not consider that index when choosing normal execution plans. This makes hidden indexes especially useful for safely testing whether an index can be removed. An administrator can hide the index and observe production query behavior as though the index were absent. If performance or plan quality degrades, the index can be unhidden immediately without rebuilding it because MongoDB has continued to maintain it in the background. Option A is incorrect because a hidden index is still an index and is not simply removed from index statistics. Option B is also incorrect; hiding an index can actually force collection scans or alternative plans if that index was previously important. Option D misunderstands the feature entirely: hiding an index has no effect on document visibility. Hidden indexes are therefore a risk-reduction mechanism for index lifecycle management, especially before dropping an index in a production environment. Option C accurately states that purpose.

Study Guide reference/topic:Indexing and Query Optimization - hidden indexes, query planner behavior, and safe index-removal testing.

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