$explain

The $explain operator provides information on the query plan. It returns a document that describes the process and indexes used to return the query. This may provide useful insight when attempting to optimize a query.

mongo shell also provides the explain() method:

db.collection.find().explain()

You can also specify the option in either of the following forms:

db.collection.find()._addSpecial( "$explain", 1 )
db.collection.find( { $query: {}, $explain: 1 } )

For details on the output, see 解释输出.

$explain runs the actual query to determine the result. Although there are some differences between running the query with $explain and running without, generally, the performance will be similar between the two. So, if the query is slow, the $explain operation is also slow.

Additionally, the $explain operation reevaluates a set of candidate query plans, which may cause the $explain operation to perform differently than a normal query. As a result, these operations generally provide an accurate account of how MongoDB would perform the query, but do not reflect the length of these queries.

To determine the performance of a particular index, you can use hint() and in conjunction with explain(), as in the following example:

db.products.find().hint( { type: 1 } ).explain()

When you run explain() with hint(), the query optimizer does not reevaluate the query plans.