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Query Optimization

Evaluate Performance of Current Operations

On this page

  • Use the Database Profiler to Evaluate Operations Against the Database
  • Use db.currentOp() to Evaluate mongod Operations
  • Use explain to Evaluate Query Performance

The following sections describe techniques for evaluating operational performance.

Use the Database Profiler to Evaluate Operations Against the Database

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, 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.

  • Are logged under the REPL component with the text applied op: <oplog entry> took <num>ms.

  • Do not depend on the log levels (either at the system or component level)

  • Do not depend on the profiling level.

  • May be affected by slowOpSampleRate, depending on your MongoDB version:

    • In MongoDB 4.2, these slow oplog entries are not affected by the slowOpSampleRate. MongoDB logs all slow oplog entries regardless of the sample rate.

    • In MongoDB 4.4 and later, these slow oplog entries are affected by the slowOpSampleRate.

The profiler does not capture slow oplog entries.

For more information, see Database Profiler.

Use db.currentOp() to Evaluate mongod Operations

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

Use explain to Evaluate Query Performance

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.

Example

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 mongosh:

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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Query Optimization