What will happen if you try to query a hidden index?

When querying a hidden index in MongoDB, the query will proceed without utilizing the hidden index. Hidden indexes are designed to remain in the database for various purposes, such as maintaining historical data or testing query performance, but they are not considered during query execution unless explicitly referenced. This allows developers to retain the index and potentially analyze its effectiveness without impacting the performance of ongoing queries.

In terms of query execution, when a hidden index is present, it does not obstruct or disrupt the process, allowing MongoDB to either use an alternate index that is not hidden or perform a collection scan to retrieve the data, depending on the nature of the query and the available indexes. This characteristic demonstrates how hidden indexes provide flexibility and do not interfere with the performance of database operations when not needed.