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Cursor Methods

cursor.explain()

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  • Definition
  • Behavior
  • Output
  • Example

Definition

cursor.explain(verbosity)

Important

mongosh Method

This page documents a mongosh method. This is not the documentation for a language-specific driver, such as Node.js.

For MongoDB API drivers, refer to the language-specific MongoDB driver documentation.

Provides information on the query plan for the db.collection.find() method.

The explain() method has the following form:

db.collection.find().explain()

The explain() method has the following parameter:

Parameter
Type
Description
verbose
string

Optional. Specifies the verbosity mode for the explain output. The mode affects the behavior of explain() and determines the amount of information to return. The possible modes are: "queryPlanner", "executionStats", and "allPlansExecution".

Default mode is "queryPlanner".

For backwards compatibility with earlier versions of cursor.explain(), MongoDB interprets true as "allPlansExecution" and false as "queryPlanner".

For more information on the modes, see Verbosity Modes.

The explain() method returns a document with the query plan and, optionally, the execution statistics.

Behavior

Verbosity Modes

The behavior of cursor.explain() and the amount of information returned depend on the verbosity mode.

queryPlanner Mode

By default, cursor.explain() runs in queryPlanner verbosity mode.

MongoDB runs the query optimizer to choose the winning plan for the operation under evaluation. cursor.explain() returns the queryPlanner information for the evaluated method.

executionStats Mode

MongoDB runs the query optimizer to choose the winning plan, executes the winning plan to completion, and returns statistics describing the execution of the winning plan.

cursor.explain() returns the queryPlanner and executionStats information for the evaluated method. However, executionStats does not provide query execution information for the rejected plans.

allPlansExecution Mode

MongoDB runs the query optimizer to choose the winning plan and executes the winning plan to completion. In "allPlansExecution" mode, MongoDB returns statistics describing the execution of the winning plan as well as statistics for the other candidate plans captured during plan selection.

cursor.explain() returns the queryPlanner and executionStats information for the evaluated method. The executionStats includes the completed query execution information for the winning plan.

If the query optimizer considered more than one plan, executionStats information also includes the partial execution information captured during the plan selection phase for both the winning and rejected candidate plans.

db.collection.explain().find()

db.collection.explain().find() is similar to db.collection.find().explain() with the following key differences:

See db.collection.explain() for more information.

Output

cursor.explain() operations can return information regarding:

  • explainVersion, the output format version (for example, "1");

  • command, which details the command being explained;

  • queryPlanner, which details the plan selected by the query optimizer and lists the rejected plans;

  • executionStats, which details the execution of the winning plan and the rejected plans;

  • serverInfo, which provides information on the MongoDB instance; and

  • serverParameters, which details internal parameters.

The verbosity mode (i.e. queryPlanner, executionStats, allPlansExecution) determines whether the results include executionStats and whether executionStats includes data captured during plan selection.

Explain output is limited by the maximum Nested Depth for BSON Documents, which is 100 levels of nesting. Explain output that exceeds the limit is truncated.

For details on the output, see Explain Results.

Example

The following example runs cursor.explain() in "executionStats" verbosity mode to return the query planning and execution information for the specified db.collection.find() operation:

db.products.find(
   { quantity: { $gt: 50 }, category: "apparel" }
).explain("executionStats")

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