What happens if the collection name provided to the $out stage already exists?

When using the $out stage in a MongoDB aggregation pipeline, if the specified collection name already exists, the existing collection is overwritten with the results of the aggregation. This means that the previous data in the collection will be deleted, and the new output from the pipeline will take its place.

This behavior is important for users because it allows for clear updates to collections without needing to manually drop a collection before outputting new data into it. The $out stage is specifically designed to streamline the process of exporting data from an aggregation pipeline directly into a MongoDB collection. By overwriting the collection, it ensures that the most current results are stored without unnecessary duplication.

Other options presented do not accurately reflect how the $out stage operates in MongoDB. For example, creating a backup or appending data would introduce complexity and is not within the functionality of the $out stage, which is focused solely on producing a definitive output. Similarly, raising an error would imply that the operation is not permissible, which is not the case, as the design of the $out stage is to allow for straightforward overwriting of existing collections whenever necessary.