In a query plan, what does 'COLLSCAN' refer to?

In the context of a query plan in MongoDB, 'COLLSCAN' refers to a collection scan, indicating that the database engine is scanning through all the documents in a collection to fulfill a query. This process involves examining each document sequentially, which can be inefficient, especially for large collections, as it doesn’t leverage any indexes that could speed up the retrieval of relevant documents.

When a query does not utilize an index that has been defined for the collection, MongoDB falls back on performing a collection scan. This is often observed in scenarios where the query conditions do not match any available indexes or where the use of an index would not significantly reduce the number of documents to inspect.

In contrast, the other choices represent different aspects of how MongoDB interacts with its data. An index scan involves examining entries in an index rather than the documents themselves, which is much faster for selective queries. Sorting operations refer to arranging data based on some criteria, and fetch operations pertain to retrieving specific documents from a dataset after having identified which documents match the query criteria.