What information does the explain() command return?

The explain() command in MongoDB is a powerful tool used to provide detailed information about how a query is executed. When invoked, it returns a comprehensive report known as the query plan. This report includes several important components, most notably the "winning plan" subsection, which outlines the specific steps the query engine will take to retrieve the requested documents. This can include details on the indexes that are utilized, any filtering that's applied, the stages involved in the query, and how documents are returned to the client.

Understanding the winning plan is critical for developers and database administrators, as it helps them optimize queries and improve application performance. The data returned by the explain() command is vital for diagnosing potential performance issues, assessing how effectively indexes are being used, and fine-tuning query structure for better efficiency.

The other choices do not encompass the breadth of information provided by the explain() command. While it may indeed include information about indexes, it does so as part of a much broader dataset regarding the overall execution strategy. Similarly, while execution time can be assessed through other means, the primary focus of the explain() command is to provide insights into the execution plan rather than simply reporting on the number of documents affected or just the query's execution time alone.