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TechTip: Real-Time Monitoring of Query Resource Consumption with Query Supervisor

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Query Supervisor gives you the ability to not only monitor for expensive and runaway queries, but to take action while the query is running.

Identifying and managing long-running and resource-intensive queries can be challenging. You can try to identify and fix these queries before they are used in your production environment, but this is not always possible. For example, you may have a user perform a long-running ad hoc query that consumes a lot of resources, or you may have a query suddenly take longer to complete than usual. It is important to be able to detect these long-running, resource-intensive queries so that you can achieve improved systems management.

Query Supervisor provides you with the ability to not only monitor for expensive and runaway queries while a query is running but to take action when a predefined query’s resource-consumption threshold is reached. For example, you can define a threshold of 90 seconds for the total elapsed time for a query to complete. If a query takes longer than 90 second to complete, Query Supervisor will interrupt the query and call a user-defined exit program that can take actions such as sending a notification message, logging details about the long-running query, or even terminating the query.

Other tools allow you to understand your query performance either before or after a query is run. For example, the Db2 for i Predictive Query Governor allows you to stop a query before it is run if the query estimate exceeds a user-specified amount of run time or temporary storage usage, while the SQL Performance Center allows you to study a resource-intensive query after it completes. Query Supervisor, however, allows you to take action while the query is running, based on the actual resources consumed.

Query Supervisor can be configured to monitor for four query resource threshold types. These thresholds are the total CPU time used by the query, the total elapsed time for the query, the temporary storage that the query uses, and the total number of I/O operations. The ADD_QUERY_THRESHOLD and REMOVE_QUERY_THRESHOLD procedures are used to configure the thresholds Query Supervisor uses.

When a query is executed, the SQL Query Engine (SQE) monitors the resource usage. If one of the thresholds that was configured is met or exceeded, the query is interrupted, and the Query Supervisor exit programs are called. The Query Supervisor exit programs are written by you and registered with the Query Supervisor Exit Point, QIBM_QQQ_QRY_SUPER. You determine what actions are taken in the exit program. Information about the query, the job, and the threshold are passed to the exit program. You can use this information to take actions such as logging information, suspending the query, or sending a message to QSYSOPR. You can also choose to terminate the query. If you do not terminate the query, the query execution will resume once the exit program completes. Multiple thresholds can be defined, but a specific threshold is processed only once during the query’s execution.

The Query Supervisor thresholds can be applied to all queries across the system, or you can choose to apply the thresholds to specific users, jobs, and subsystems. The procedure ADD_QUERY_THRESHOLD not only defines the thresholds but also further configures Query Supervisor by including or excluding users from being supervised, specifying which subsystem to supervise, or specifying the minimum time between calls to the Query Supervisor exit program. For example, to set a threshold value for total CPU time of 90 seconds for all jobs except those running with the ‘BATCHUSR’ user profile, run the following call to the procedure:

CALL QSYS2.ADD_QUERY_THRESHOLD(THRESHOLD_NAME         => 'Maximum CPU',

                               THRESHOLD_TYPE         => 'CPU TIME',

                               THRESHOLD_VALUE       => 90,

                               EXCLUDE_USERS         => 'BATCHUSR');

You define what the exit program does. The exit program will interrupt the query, so the exit program should be as simple as possible. IBM has provided different example exit programs that you can use to send messages to QSYSOPR, terminate the query, or log information about the query. You can use these examples as a starting point and modify them to meet your unique needs.

It is important to understand and protect your production environment from long-running queries. Query Supervisor gives you the ability to not only monitor for these expensive queries, but also take action while the query is running. Query Supervisor is available with 7.3 TR10, 7.4 TR4, and the 7.5 Base release.

For more information about Query Supervisor, see https://www.ibm.com/docs/en/i/7.4?topic=tools-query-supervisor.

Sarah Mackenzie is a Db2 for i Senior Software Engineer. She has worked on the IBM i development team since she joined IBM in 2012. During that time, she has focused on both database and query and has worked on the design, development, and support of new enhancements for IBM i such as Temporal Tables, Db2 Mirror, and Geospatial Analytics with Watson. Sarah is also a speaker at many industry events.

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