Measure Index Use

Get Index Access Information with $indexStats

Use the $indexStats aggregation stage to get statistics regarding the use of each index for a collection. For example, the following aggregation operation returns statistics on the index use on the orders collection:

db.orders.aggregate( [ { $indexStats: { } } ] )

Return Query Plan with explain()

Use the db.collection.explain() or the cursor.explain() method in executionStats mode to return statistics about the query process, including the index used, the number of documents scanned, and the time the query takes to process in milliseconds.

Run db.collection.explain() or the cursor.explain() method in allPlansExecution mode to view partial execution statistics collected during plan selection.

Control Index Use with hint()

To force MongoDB to use a particular index for a db.collection.find() operation, specify the index with the hint() method. Append the hint() method to the find() method. Consider the following example:

db.people.find(
   { name: "John Doe", zipcode: { $gt: "63000" } }
).hint( { zipcode: 1 } )

To view the execution statistics for a specific index, append to the db.collection.find() the hint() method followed by cursor.explain(), e.g.:

db.people.find(
   { name: "John Doe", zipcode: { $gt: "63000" } }
).hint( { zipcode: 1 } ).explain("executionStats")

Or, append hint() method to db.collection.explain().find():

db.people.explain("executionStats").find(
   { name: "John Doe", zipcode: { $gt: "63000" } }
).hint( { zipcode: 1 } )

Specify the $natural operator to the hint() method to prevent MongoDB from using any index:

db.people.find(
   { name: "John Doe", zipcode: { $gt: "63000" } }
).hint( { $natural: 1 } )

Index Metrics

In addition to the $indexStats aggregation stage, MongoDB provides various index statistics that you may want to consider when analyzing index use for your database:

• In the output of serverStatus:

  • metrics.queryExecutor.scanned

  • metrics.operation.scanAndOrder

• In the output of collStats:

  • totalIndexSize

  • indexSizes

• In the output of dbStats:

  • dbStats.indexes

  • dbStats.indexSize