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Monitor and Improve Slow Queries

On this page

  • Common Reasons for Slow Queries
  • Required Access
  • Configure the Slow Query Threshold
  • Index Considerations
  • Access Performance Advisor
  • Index Suggestions
  • Create Suggested Indexes

Only available on M10+ clusters and serverless instances

The Performance Advisor monitors queries that MongoDB considers slow and suggests new indexes to improve query performance. The threshold for slow queries varies based on the average time of operations on your cluster to provide recommendations pertinent to your workload.

Recommended indexes are accompanied by sample queries, grouped by query shape, that were run against a collection that would benefit from the suggested index. The Performance Advisor doesn't negatively affect the performance of your Atlas clusters.

You can also monitor collection-level query latency with Namespace Insights and query performance with the Query Profiler.

Note

If a query is slow, common reasons include:

  • The query is unsupported by your current indexes.

  • Some documents in your collection have large array fields that are costly to search and index.

  • One query retrieves information from multiple collections with $lookup.

To view collections with slow queries and see suggested indexes, you must have Project Read Only access or higher to the project.

To view field values in a sample query in the Performance Advisor, you must have Project Data Access Read/Write access or higher to the project.

To enable or disable the Atlas-managed slow operation threshold, you must have Project Owner access to the project. Users with Organization Owner access must add themselves to the project as a Project Owner.

By default, Atlas dynamically adjusts your slow query threshold based on the execution time of operations across your cluster. However, you can opt out of this feature and instead use a fixed slow query threshold of 100 milliseconds. You can disable the Atlas-managed slow operation threshold with the Atlas CLI, Atlas Administration API, or Atlas UI.

Note

Atlas clusters with Atlas Search enabled don't support the Atlas-managed slow query operation threshold.

For M0, M2, M5 clusters and serverless instances, Atlas disables the Atlas-managed slow query operation threshold by default and you can't enable it.

Important

By default, Atlas dynamically adjusts your slow query threshold based on the execution time of operations across your cluster. If you disable the Atlas-managed slow query threshold, it no longer dynamically adjusts. MongoDB defaults the fixed slow query threshold to 100 milliseconds. We don't recommend that you set the fixed slow query threshold lower than 100 milliseconds.

To disable the Atlas-managed slow operation threshold and use a fixed threshold of 100 milliseconds:

To disable the Atlas-managed slow operation threshold for your project using the Atlas CLI, run the following command:

atlas performanceAdvisor slowOperationThreshold disable [options]

To learn more about the command syntax and parameters, see the Atlas CLI documentation for atlas performanceAdvisor slowOperationThreshold disable.

Tip

See: Related Links

In the Project Settings for the current project, toggle Managed Slow Operations to Off.

Atlas enables the Atlas-managed slow operation threshold by default. To re-enable the Atlas-managed slow operation threshold that you previously disabled:

To enable the Atlas-managed slow operation threshold for your project using the Atlas CLI, run the following command:

atlas performanceAdvisor slowOperationThreshold enable [options]

To learn more about the command syntax and parameters, see the Atlas CLI documentation for atlas performanceAdvisor slowOperationThreshold enable.

In the Project Settings for the current project, toggle Managed Slow Operations to On.

Indexes improve read performance, but a large number of indexes can negatively impact write performance since indexes must be updated during writes. If your collection already has several indexes, consider this tradeoff of read and write performance when deciding whether to create new indexes. Examine whether a query for such a collection can be modified to take advantage of existing indexes, as well as whether a query occurs often enough to justify the cost of a new index.

To return up to 20 namespaces in <database>.<collection> format for collections experiencing slow queries using the Atlas CLI, run the following command:

atlas performanceAdvisor namespaces list [options]

To learn more about the command syntax and parameters, see the Atlas CLI documentation for atlas performanceAdvisor namespaces list.

To return query log line items for slow queries that the Performance Advisor and Query Profiler identify using the Atlas CLI, run the following command:

atlas performanceAdvisor slowQueryLogs list [options]

To learn more about the command syntax and parameters, see the Atlas CLI documentation for atlas performanceAdvisor slowQueryLogs list.

To return suggested indexes for collections experiencing slow queries using the Atlas CLI, run the following command:

atlas performanceAdvisor suggestedIndexes list [options]

To learn more about the command syntax and parameters, see the Atlas CLI documentation for atlas performanceAdvisor suggestedIndexes list.

To access the Performance Advisor using the Atlas UI:

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  1. If it's not already displayed, select the organization that contains your desired project from the Organizations menu in the navigation bar.

  2. If it's not already displayed, select your desired project from the Projects menu in the navigation bar.

  3. If the Clusters page is not already displayed, click Database in the sidebar.

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If the replica set resides in a sharded cluster, first click the sharded cluster containing the replica set.

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  1. If it's not already displayed, select the organization that contains your desired project from the Organizations menu in the navigation bar.

  2. If it's not already displayed, select your desired project from the Projects menu in the navigation bar.

  3. If the Clusters page is not already displayed, click Database in the sidebar.

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The Performance Advisor displays up to 20 query shapes across all collections in the cluster and suggested indexes for those shapes. The Performance Advisor ranks the indexes according to their Impact, which indicates High or Medium based on the total wasted bytes read. To learn more about index ranking, see Review Index Ranking.

The Performance Advisor ranks the indexes that it suggests according to their Impact, which indicates High or Medium based on the total wasted bytes read. To learn more about how the Performance Advisor ranks indexes, see Review Index Ranking.

To learn how to create indexes that the Performance Advisor suggests, see Create Suggested Indexes.

Each index that the Performance Advisor suggests contains the following metrics. These metrics apply specifically to queries which would be improved by the index:

Metric
Description
Execution Count
Number of queries executed per hour which would be improved.
Average Execution Time
Current average execution time in milliseconds for affected queries.
Average Query Targeting
Average number of documents read per document returned by affected queries. A higher query targeting score indicates a greater degree of inefficiency. For more information on query targeting, see Query Targeting.
In Memory Sort
Current number of affected queries per hour that needed to be sorted in memory.
Average Docs Scanned
Average number of documents scanned.
Average Docs Returned
Average number of documents returned.
Average Object Size
Average object size.

For each suggested index, the Performance Advisor shows the most commonly executed query shapes that the index would improve. For each query shape, the Performance Advisor displays the following metrics:

Metric
Description
Execution Count
Number of queries executed per hour which match the query shape.
Average Execution Time
Average execution time in milliseconds for queries which match the query shape.
Average Query Targeting
Average number of documents read for every document returned by matching queries. A higher query targeting score indicates a greater degree of inefficiency. For more information on query targeting, see Query Targeting.
Average Docs Scanned
Average number of documents scanned.
Average Docs Returned
Average number of documents returned.

The Performance Advisor also shows each executed sample query that matches the query shape, with specific metrics for that query.

Each index suggestion includes an Average Query Targeting score indicating how many documents were read for every document returned for the index's corresponding query shapes. A score of 1 represents very efficient query shapes because every document read matched the query and was returned with the query results. All suggested indexes represent an opportunity to improve query performance.

By default, the Performance Advisor suggests indexes for all clusters in the deployment. To only show suggested indexes from a specific collection, use the Collection dropdown at the top of the Performance Advisor.

You can also adjust the time range the Performance Advisor takes into account when suggesting indexes by using the Time Range dropdown at the top of the Performance Advisor.

The Performance Advisor can't suggest indexes for MongoDB databases configured to use the ctime timestamp format. As a workaround, set the timestamp format for such databases to either iso8601-utc or iso8601-local. To learn more about timestamp formats, see mongod --timeStampFormat.

The Performance Advisor analyzes up to 200,000 of your cluster's most recent log lines.

If a cluster experiences an activity spike and generates an extremely large quantity of log messages, Atlas may stop collecting and storing new logs for a period of time.

Note

Log analysis rate limits apply only to the Performance Advisor UI, the Query Insights UI, the Access Tracking UI, and the Atlas Search Query Analytics UI. Downloadable log files are always complete.

The Performance Advisor doesn't provide performance suggestions for time-series collections.

The Performance Advisor includes a user feedback button for Index Suggestions. Atlas hides this button for serverless instances.

You can create indexes suggested by the Performance Advisor directly within the Performance Advisor itself. When you create indexes, keep the ratio of reads to writes on the target collection in mind. Indexes come with a performance cost, but are more than worth the cost for frequent queries on large data sets. To learn more about indexing strategies, see Indexing Strategies.

  • You can't create indexes through the Performance Advisor if Data Explorer is disabled for your project. You can still view the Performance Advisor recommendations, but you must create those indexes from mongosh.

  • You can only create one index at a time through the Performance Advisor. If you want to create more simultaneously, you can do so using the Atlas UI, a driver, or the shell

  • Atlas always creates indexes for entire clusters. If you create an index while viewing the Performance Advisor for a single shard in a sharded cluster, Atlas creates that index for the entire sharded cluster.

To create a suggested index:

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The Performance Advisor opens the Create Index dialog box and prepopulates the Fields based on the index you selected.

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{ <option1>: <value1>, ... }

Example

The following options document specifies the unique option and the name for the index:

{ unique: true, name: "myUniqueIndex" }
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Use collation to specify language-specific rules for string comparison, such as rules for lettercase and accent marks. The collation document contains a locale field which indicates the ICU Locale code, and may contain other fields to define collation behavior.

Example

The following collation option document specifies a locale value of fr for a French language collation:

{ "locale": "fr" }

To review the list of locales that MongoDB collation supports, see the list of languages and locales. To learn more about collation options, including which are enabled by default for each locale, see Collation in the MongoDB manual.

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Important

Rolling index builds succeed only when they meet certain conditions. To ensure your index build succeeds, avoid the following design patterns that commonly trigger a restart loop:

Note

the Atlas UI doesn't support building indexes with a rolling build for M0 free clusters and M2/M5 shared clusters. You can't build indexes with a rolling build for serverless instances.

For workloads which cannot tolerate performance decrease due to index builds, consider building indexes in a rolling fashion.

To maintain cluster availability:

  • Atlas removes one node from the cluster at a time starting with a secondary.

  • More than one node can go down at a time, but Atlas always keeps a majority of the nodes online.

Atlas automatically cancels rolling index builds that don't succeed on all nodes. When a rolling index build completes on some nodes, but fails on others, Atlas cancels the build and removes the index from any nodes that it was successfully built on.

In the event of a rolling index build cancellation, Atlas generates an activity feed event and sends a notification email to the project owner with the following information:

  • Name of the cluster on which the rolling index build failed

  • Namespace on which the rolling index build failed

  • Project that contains the cluster and namespace

  • Organization that contains the project

  • Link to the activity feed event

To learn more about rebuilding indexes, see Build Indexes on Replica Sets.

Note

Unique index options are incompatible with building indexes in a rolling fashion. If you specify unique in the Options pane, Atlas rejects your configuration with an error message.

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Important

When an index build completes, Atlas generates an activity feed event and sends a notification email to the project owner with the following information:

  • Completion date of the index build

  • Name of the cluster on which the index build completed

  • Namespace on which the index build completed

  • Project containing the cluster and namespace

  • Organization containing the project

  • Link to the activity feed event

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Analyze Slow Queries

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Index Ranking