$explain¶

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Behavior

$explain¶

Note

Deprecated in the mongo Shell since v3.0: Starting in v3.2, the $explain operator is deprecated in the mongo shell. In the mongo shell, use db.collection.explain() or cursor.explain() instead.

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. For details on the output, see cursor.explain().

You can specify the $explain operator in either of the following forms:

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

In the mongo shell, you also can retrieve query plan information through the explain() method:

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db.collection.find().explain()

Behavior¶

$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.