Fix Query Issues
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Query Targeting alerts indicate inefficient queries.
Alert Conditions
You can configure the following alert conditions in the project-level alert settings page to trigger alerts.
Query Targeting: Scanned Objects / Returned occurs if the
number of documents examined to fulfill a query relative to the actual
number of returned documents meets or exceeds a user-defined threshold.
The default is 1000, which means that a query must scan more than 1000
documents for each document returned to trigger the alert.
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.
Ideally, the ratio of scanned documents to returned documents should be close to 1. A high ratio negatively impacts query performance.
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.
Common Triggers
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.
Fix the Immediate Problem
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.
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
mongodinstance in your Atlas clusters. - The cursor.explain()
command for
mongoshprovides performance details for all queries. The Data Profiler records operations that Atlas considers slow when compared to average execution time for all operations on your cluster.
NoteEnabling the Database Profiler incurs a performance overhead.
Implement a Long-Term Solution
Refer to the following for more information on query performance:
Monitor Your Progress
Atlas provides two methods to visualize query targeting:
- Query Targeting metrics, which highlight high ratios of objects scanned to objects returned.
- The Query Profiler, which describes specific inefficient queries executed on the cluster.
Query Targeting Metrics
You can view historical metrics to help you visualize the query performance of your cluster. To view Query Targeting metrics in the Atlas UI:
- Click Databases in the top-left corner of Atlas.
- Click View Monitoring on the dashboard for the database deployment.
- 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. |
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.
You can also view Query Targeting ratios of operations in real-time using the Real-Time Performance Panel.
Query Profiler
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.
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.
To access the Query Profiler:
- Click Databases in the top-left corner of Atlas.
- Click View Monitoring on the dashboard for the database deployment.
- Click the Profiler tab.