$explain¶

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

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

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

You also can specify $explain through the explain() method in the mongo shell:

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.

参见

explain()
Optimization Strategies for MongoDB page for information regarding optimization strategies.
Analyze Performance of Database Operations tutorial for information regarding the database profile.
Current Operation Reporting

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