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Fix Query Issues

On this page

  • Alert Conditions
  • Common Triggers
  • Fix the Immediate Problem
  • Implement a Long-Term Solution
  • Monitor Your Progress

Query Targeting alerts often indicate inefficient queries.

You can configure the following alert conditions in the project-level alert settings page to trigger alerts.

Query Targeting: Scanned Objects / Returned alerts are triggered when the average number of documents scanned relative to the average number of documents returned server-wide across all operations during a sampling period exceeds a defined threshold. The default alert uses a 1000:1 threshold.

Ideally, the ratio of scanned documents to returned documents should be close to 1. A high ratio negatively impacts query performance.

Query Targeting: Scanned / Returned occurs if the number of index keys examined to fulfill a query relative to the actual number of returned documents meets or exceeds a user-defined threshold. This alert is not enabled by default.

Example

The following mongod log entry shows statistics generated from an inefficient query:

<Timestamp> COMMAND <query>
planSummary: COLLSCAN keysExamined:0
docsExamined: 10000 cursorExhausted:1 numYields:234
nreturned:4 protocol:op_query 358ms

This query scanned 10,000 documents and returned only 4 for a ratio of 2500, which is highly inefficient. No index keys were examined, so MongoDB scanned all documents in the collection, known as a collection scan.

The query targeting alert typically occurs when there is no index to support a query or queries or when an existing index only partially supports a query or queries.

Note

The change streams cursors that the Atlas Search process (mongot) uses to keep Atlas Search indexes updated can contribute to the query targeting ratio and trigger query targeting alerts if the ratio is high.

Add one or more indexes to better serve the inefficient queries.

The Performance Advisor provides the easiest and quickest way to create an index. The Performance Advisor monitors queries that MongoDB considers slow and recommends indexes to improve performance. Atlas dynamically adjusts your slow query threshold based on the execution time of operations across your cluster.

Click Create Index on a slow query for instructions on how to create the recommended index.

Note

It is possible to receive a Query Targeting alert for an inefficient query without receiving index suggestions from the Performance Advisor if the query exceeds the slow query threshold and the ratio of scanned to returned documents is greater than the threshold specified in the alert.

In addition, you can use the following resources to determine which query generated the alert:

  • The Real-Time Performance Panel monitors and displays current network traffic and database operations on machines hosting MongoDB in your Atlas clusters.

  • The MongoDB logs maintain an account of activity, including queries, for each mongod instance in your Atlas clusters.

  • The cursor.explain() command for mongosh provides performance details for all queries.

  • Namespace Insights monitors collection-level query latency.

  • The Atlas Query Profiler records operations that Atlas considers slow when compared to average execution time for all operations on your cluster.

Refer to the following for more information on query performance:

Atlas provides the following methods to visualize query targeting:

  • Query Targeting metrics, which highlight high ratios of objects scanned to objects returned.

  • Namespace Insights, which monitors collection-level query latency.

  • The Query Profiler, which describes specific inefficient queries executed on the cluster.

You can view historical metrics to help you visualize the query performance of your cluster. To view Query Targeting metrics in the Atlas UI:

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

  2. If it is 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.

2
  1. Click View Monitoring on the dashboard for the cluster.

  2. On the Metrics page, click the Add Chart dropdown menu and select Query Targeting.

The Query Targeting chart displays the following metrics for queries executed on the server:

Metric
Description
Scanned Objects / Returned
Indicates the average number of documents examined relative to the average number of returned documents.
Scanned / Returned
Indicates the number of index keys examined to fulfill a query relative to the actual number of returned documents.

Note

The change streams cursors that the Atlas Search process (mongot) uses to keep Atlas Search indexes updated can contribute to the query targeting ratio and trigger query targeting alerts if the ratio is high.

If either of these metrics exceed the user-defined threshold, Atlas generates the corresponding Query Targeting: Scanned Objects / Returned or Query Targeting: Scanned / Returned alert.

Note

You can also view Query Targeting ratios of operations in real-time using the Real-Time Performance Panel.

Namespace Insights monitors collection-level query latency. You can view query latency metrics and statistics for certain hosts and operation types. Manage pinned namespaces and choose up to five namespaces to show in the corresponding query latency charts.

To access Namespace Insights:

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

  2. If it is 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.

2
  1. Click View Monitoring on the dashboard for the cluster.

  2. Click the Query Insights tab.

  3. Click the Namespace Insights tab.

The Query Profiler contains several metrics you can use to pinpoint specific inefficient queries. You can visualize up to the past 24 hours of query operations. The Query Profiler can show the Examined : Returned Ratio (index keys examined to documents returned) of logged queries, which might help you identify the queries that triggered a Query Targeting: Scanned / Returned alert. The chart shows the number of index keys examined to fulfill a query relative to the actual number of returned documents.

Note

The default Query Targeting: Scanned Objects / Returned alert ratio differs slightly. The ratio of the average number of documents scanned to the average number of documents returned during a sampling period triggers this alert.

Atlas might not log the individual operations that contribute to the Query Targeting ratios due to automatically set thresholds. However, you can still use the Query Profiler and Query Targeting metrics to analyze and optimize query performance.

To access the Query Profiler:

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

  2. If it is 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.

2
  1. Click View Monitoring on the dashboard for the cluster.

  2. Click the Query Insights tab.

  3. Click the Query Profiler tab.

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