explain

Definition

• explain

New in version 3.0.

The explain command provides information on theexecution of the following commands: aggregate,count, distinct, find,findAndModify, delete, andupdate.

Although MongoDB provides the explain command, thepreferred method for running explain is to use thedb.collection.explain() and cursor.explain() helpers.

The explain command has the following syntax:

{
   explain: <command>,
   verbosity: <string>
}

The command takes the following fields:

FieldTypeDescriptionexplaindocumentA document specifying the command for which to return the executioninformation. For details on the specific command document, see aggregate, count,distinct, find,findAndModify, delete, and update.verbositystringOptional. A string specifying the mode in which to run explain.The mode affects the behavior of explain and determinesthe amount of information to return.

The possible modes are:

• "queryPlanner"
• "executionStats"
• "allPlansExecution" (Default)For more information on the modes, see explain behavior.

Behavior

Verbosity Modes

The behavior of explain and the amount of informationreturned depend on the verbosity mode.

• queryPlanner Mode
• executionStats Mode
• allPlansExecution Mode (Default)

MongoDB runs the query optimizer to choosethe winning plan for the operation under evaluation. explain returnsthe queryPlanner information for the evaluated<command>.

MongoDB runs the query optimizer to choosethe winning plan, executes the winning plan to completion, and returnsstatistics describing the execution of the winning plan.

For write operations, explain returns information aboutthe update or delete operations that would be performed, but doesnot apply the modifications to the database.

explain returns the queryPlanner andexecutionStats information for the evaluated<command>. However, executionStats does notprovide query execution information for the rejected plans.

By default, explain runs in"allPlansExecution" verbosity mode.

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

For write operations, explain returns information aboutthe update or delete operations that would be performed, but doesnot apply the modifications to the database.

explain returns the queryPlanner andexecutionStats information for the evaluated<command>. The executionStats includes thecompleted 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 rejectedcandidate plans.

Explain and Write Operations

For write operations, the explain command returnsinformation about the write operation that would be performed but doesnot actually modify the database.

Restrictions

Starting in MongoDB 4.2, you cannot run the explaincommand/db.collection.explain() in executionStats modeor 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 allPlansExecutionmode but without the $out stage to return informationfor the stages that precede the $out stage.

Examples

queryPlanner Mode

The following explain command runs in "queryPlanner"verbosity mode to return the query planning information for acount command:

db.runCommand(
   {
     explain: { count: "products", query: { quantity: { $gt: 50 } } },
     verbosity: "queryPlanner"
   }
)

executionStats Mode

The following explain operation runs in "executionStats"verbosity mode to return the query planning and execution informationfor a count command:

db.runCommand(
   {
      explain: { count: "products", query: { quantity: { $gt: 50 } } },
      verbosity: "executionStats"
   }
)

allPlansExecution Mode

By default, explain runs in "allPlansExecution" verbositymode. The following explain command returns thequeryPlanner and executionStats forall considered plans for an update command:

Note

The execution of this explain will not modify data but runs thequery predicate of the update operation. For candidate plans,MongoDB returns the execution information captured during theplan selection phase.

db.runCommand(
   {
     explain: {
        update: "products",
        updates: [
           {
               q: { quantity: 1057, category: "apparel" },
               u: { $set: { reorder: true } }
           }
        ]
     }
   }
)

Output

explain operations can return information regarding:

• queryPlanner, which details the plan selected by thequery optimizer and lists the rejectedplans;
• executionStats, which details the execution of the winningplan and the rejected plans; and
• serverInfo, which provides information on theMongoDB instance.

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

For details on the output, see Explain Results.