MongoDB with drivers
Description
db.collection.explain()Returns information on the query plan for the following methods:
Returns information on
mapReduce().To use
db.collection.explain(), append one of the aforementioned methods todb.collection.explain():db.collection.explain().<method(...)> For example,
db.products.explain().remove( { category: "apparel" }, { justOne: true } ) For more examples, see Examples. See also db.collection.explain().help().
Parameters
The db.collection.explain() method has the following parameter:
Parameter | Type | Description |
|---|---|---|
| string | Optional. Specifies the verbosity mode for the explain output. The mode affects the behavior of
For backwards compatibility with earlier versions of For more information on the modes, see Verbosity Modes. |
Note
db.collection.explain() defaults to queryPlanner, unlike the explain command, which defaults to allPlansExecution.
Required Access
To use explain, you must have permission to run the underlying command.
Behavior
Note
Using explain ignores all existing plan cache entries and prevents the MongoDB query planner from creating a new plan cache entry.
Verbosity Modes
The behavior of db.collection.explain() and the amount of information returned depend on the verbosity mode.
By default, db.collection.explain() runs in queryPlanner verbosity mode.
MongoDB runs the query optimizer to choose the winning plan for the operation under evaluation. db.collection.explain() returns the queryPlanner information for the evaluated method.
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.
For write operations, db.collection.explain() returns information about the update or delete operations that would be performed, but does not apply the modifications to the database.
db.collection.explain() returns the queryPlanner and executionStats information for the evaluated method. However, executionStats does not provide query execution information for the rejected plans.
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.
For write operations, db.collection.explain() returns information about the update or delete operations that would be performed, but does not apply the modifications to the database.
db.collection.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.
Explain and Write Operations
For write operations, db.collection.explain() returns information about the write operation that would be performed but does not actually modify the database.
Restrictions
You cannot run the explain command/db.collection.explain() in executionStats mode or allPlansExecution mode for an aggregation pipeline that contains the $out stage. Instead, you can either:
run the explain in
queryPlannermode or
explain() Mechanics
The db.collection.explain() method wraps the explain command and is the preferred way to run explain.
db.collection.explain().find() is similar to db.collection.find().explain() with the following key differences:
The
db.collection.explain().find()construct allows for the additional chaining of query modifiers. For list of query modifiers, see db.collection.explain().find().help().The
db.collection.find().explain()returns theexplain()information on the query plan.
db.collection.explain().aggregate() is equivalent to passing the explain option to the db.collection.aggregate() method.
help()
To see the list of operations supported by db.collection.explain(), run:
db.collection.explain().help()
db.collection.explain().find() returns a cursor, which allows for the chaining of query modifiers. To see the list of query modifiers supported by db.collection.explain().find() as well as cursor-related methods, run:
db.collection.explain().find().help()
You can chain multiple modifiers to db.collection.explain().find(). For an example, see Explain find() with Modifiers.
Output
db.collection.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.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.
Examples
queryPlanner Mode
By default, db.collection.explain() runs in "queryPlanner" verbosity mode.
The following example runs db.collection.explain() in "queryPlanner" verbosity mode to return the query planning information for the specified count() operation:
db.products.explain().count( { quantity: { $gt: 50 } } )
executionStats Mode
The following example runs db.collection.explain() in "executionStats" verbosity mode to return the query planning and execution information for the specified find() operation:
db.products.explain("executionStats").find( { quantity: { $gt: 50 }, category: "apparel" } )
allPlansExecution Mode
The following example runs db.collection.explain() in "allPlansExecution" verbosity mode. db.collection.explain() returns the queryPlanner and executionStats for all considered plans for the specified findAndModify() operation:
Note
The execution of this explain will not modify data but runs the query predicate of the update operation. For candidate plans, MongoDB returns the execution information captured during the plan selection phase.
db.products.explain( "allPlansExecution" ).findAndModify( { query: { name: "Tom", state: "active", rating: { $gt: 10 } }, sort: { rating: 1 }, update: { $inc: { score: 1 } } } )
Explain find() with Modifiers
db.collection.explain().find() construct allows for the chaining of query modifiers. For example, the following operation provides information on the find() method with sort() and hint() query modifiers.
db.products.explain("executionStats").find( { quantity: { $gt: 50 }, category: "apparel" } ).sort( { quantity: -1 } ).hint( { category: 1, quantity: -1 } )
For a list of query modifiers available, run the following in mongosh:
db.collection.explain().find().help()