What does 'SORT' indicate in a query plan?

In a query plan, 'SORT' indicates that documents are being sorted in memory. This process involves arranging the fields of documents based on the specified sort criteria, which can help in delivering results in a particular order that matches the query's requirements. This sorting can occur either when retrieving documents matching the query or when performing an aggregation operation.

Sorting in memory usually refers to processes where data is temporarily held in memory during the execution of the query, which may lead to performance considerations if the dataset is large enough to exceed available memory. MongoDB optimizes how data is sorted and may use indexes to improve performance, but the 'SORT' in the query plan specifically denotes that an in-memory sorting operation is taking place.