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Monitor and Improve Slow Queries
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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
Common Reasons for Slow Queries
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.
Required Access
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.
Configure the Slow Query Threshold
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.
Disable the Atlas-Managed Slow Operation Threshold
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:
Enable the Atlas-Managed Slow Operation Threshold
Atlas enables the Atlas-managed slow operation threshold by default. To re-enable the Atlas-managed slow operation threshold that you previously disabled:
Index Considerations
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.
Access Performance Advisor
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.
Index Suggestions
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.
Index Metrics
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. |
Sample Queries
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.
Query Targeting
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.
Filter Index Suggestions
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.
Limitations of Index Suggestions
Timestamp Format
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.
Log Size
The Performance Advisor analyzes up to 200,000 of your cluster's most recent log lines.
Log Quantity
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.
Time-Series Collections
The Performance Advisor doesn't provide performance suggestions for time-series collections.
User Feedback
The Performance Advisor includes a user feedback button for Index Suggestions. Atlas hides this button for serverless instances.
Create Suggested Indexes
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.
Behavior and Limitations
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 database deployments. 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.
Procedure
To create a suggested index:
For the index you want to create, click Create Index.
The Performance Advisor opens the Create Index dialog and prepopulates the Fields based on the index you selected.
(Optional) Specify the index options.
{ <option1>: <value1>, ... }
Example
The following options document specifies the unique option and
the name for the index:
{ unique: true, name: "myUniqueIndex" }
(Optional) Set the Collation options.
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.
(Optional) Enable building indexes in a rolling fashion.
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:
Index key exceeds the index key limit
Index name already exists
Index on more than one array field
Index on collection that has the maximum number of text indexes
Text index on collection that has the maximum number of text indexes
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.
Click Review.
In the Confirm Operation dialog, confirm your index.
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