Evaluate Performance of Current Operations

The following sections describe techniques for evaluating operational performance.

MongoDB provides a database profiler that shows performance characteristics of each operation against the database. Use the profiler to locate any queries or write operations that are running slow. You can use this information, for example, to determine what indexes to create.

Starting in MongoDB 4.2, the profiler entries and the diagnostic log messages (i.e. mongod/mongos log messages) for read/write operations include:

  • queryHash to help identify slow queries with the same query shape.
  • planCacheKey to provide more insight into the query plan cache for slow queries.

Starting in version 4.2 (also available starting in 4.0.6), secondary members of a replica set now log oplog entries that take longer than the slow operation threshold to apply. These slow oplog messages are logged for the secondaries in the diagnostic log under the REPL component with the text applied op: <oplog entry> took <num>ms. These slow oplog entries depend only on the slow operation threshold. They do not depend on the log levels (either at the system or component level), or the profiling level, or the slow operation sample rate. The profiler does not capture slow oplog entries.

For more information, see Database Profiler.

The db.currentOp() method reports on current operations running on a mongod instance.

The cursor.explain() and db.collection.explain() methods return information on a query execution, such as the index MongoDB selected to fulfill the query and execution statistics. You can run the methods in queryPlanner mode, executionStats mode, or allPlansExecution mode to control the amount of information returned.


To use cursor.explain() on a query for documents matching the expression { a: 1 }, in the collection named records, use an operation that resembles the following in the mongo shell:

db.records.find( { a: 1 } ).explain("executionStats")

Starting in MongoDB 4.2, the explain output includes:

For more information, see Explain Results, cursor.explain(), db.collection.explain(), and Analyze Query Performance.

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