Using an Explain Plan

Below is the code used to run the explain() method with mongosh to return an explain plan. An explain plan is a report that gives us information about possible plans for executing a pipeline, including the winning plan that MongoDB selects.

Code

explain() Method

Use the explain() method, chained with the aggregate() method to return the explain plan in mongosh.

db.collection.explain().aggregate()

Aggregation Pipeline

The output of this aggregation pipeline tells us which book genre received the highest user ratings over the last five years. The explain() method will generate an explain plan for this aggregation pipeline.

The $match stage filters for books published after 2020.

The $group stage groups documents by genre and calculates the average rating for the genre using the $avg operator.

The $sort stage sorts the genre_average_rating field in descending order to show the top genres by average rating.

db.books.explain().aggregate(
    [
      {
        $match: {
          date_of_original_publication: {
            $gte: '2020'
          }
        }
      },
      {
        $group: {
          _id: '$genre',
          genre_average_rating: {
            $avg: '$average_rating'
          }
        }
      },
      { $sort: { genre_average_rating: -1 } }
    ]
  );