db.collection.explain() (mongosh method)

Verbosity Modes

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

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 queryPlanner mode or

• run the explain in executionStats mode or allPlansExecution mode but without the $out stage to return information for the stages that precede the $out stage.

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 the explain() 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.

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:

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()

Access the explain() Results with finish() Method

The db.collection.explain().find() returns the explain() results. If run interactively in mongosh you can use the .finish() method to access the query plan:

db.products.explain().find( { category: "apparel" } ).finish().queryPlanner.winningPlan;