Measure Index Use¶

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Synopsis
Operations

Synopsis¶

Query performance is a good general indicator of index use; however, for more precise insight into index use, MongoDB provides a number of tools that allow you to study query operations and observe index use for your database.

Operations¶

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.

db.collection.explain() provides information on the execution of other operations, such as db.collection.update(). See db.collection.explain() for details.

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 } )

Instance Index Use Reporting¶

MongoDB provides a number of metrics of index use and operation 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

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