Best practices for Aurora PostgreSQL query plan management
Query plan management lets you control how and when query execution plans change. As a DBA, your main goals when using QPM include preventing regressions when there are changes to your database, and controlling whether to allow the optimizer to use a new plan. In the following, you can find some recommended best practices for using query plan management. Proactive and reactive plan management approaches differ in how and when new plans get approved for use.
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Proactive plan management to help prevent performance regression
To prevent plan performance regressions from occurring, you evolve plan baselines by running a procedure that compares the performance of newly discovered plans to the performance of the existing baseline of Approved plans, and then automatically approves the fastest set of plans as the new baseline. In this way, the baseline of plans improves over time as faster plans are discovered.
In a development environment, identify the SQL statements that have the greatest impact on performance or system throughput. Then capture the plans for these statements as described in Manually capturing plans for specific SQL statements and Automatically capturing plans.
Export the captured plans from the development environment and import them into the production environment. For more information, see Exporting and importing plans.
In production, run your application and enforce the use of approved managed plans. For more information, see Using Aurora PostgreSQL managed plans. While the application runs, also add new plans as the optimizer discovers them. For more information, see Automatically capturing plans.
Analyze the unapproved plans and approve those that perform well. For more information, see Evaluating plan performance.
While your application continues to run, the optimizer begins to use the new plans as appropriate.
Ensuring plan stability after a major version upgrade
Each major version of PostgreSQL includes enhancements and changes to the query optimizer that are designed to improve performance. However, query execution plans generated by the optimizer in earlier versions might cause performance regressions in newer upgraded versions. You can use query plan management to resolve these performance issues and to ensure plan stability after a major version upgrade.
The optimizer always uses a minimum-cost Approved plan, even if more than one Approved plan for the same statement exists. After an upgrade the optimizer might discover new plans but they will be saved as Unapproved plans. These plans are performed only if approved using the reactive style of plan management with the unapproved_plan_execution_threshold parameter. You can maximize plan stability using the proactive style of plan management with the evolve_plan_baselines parameter. This compares the performance of the new plans to the old plans and approves or rejects plans that are at least 10% faster than the next best plan.
After upgrading, you can use the
evolve_plan_baselines function to compare plan performance before
and after the upgrade using your query parameter bindings. The following steps assume that you have been using approved managed plans in your production
environment, as detailed in Using Aurora PostgreSQL managed plans.
Before upgrading, run your application with the query plan manager running. While the application runs, add new plans as the optimizer discovers them. For more information, see Automatically capturing plans.
Evaluate each plan's performance. For more information, see Evaluating plan performance.
After upgrading, analyze your approved plans again using the
evolve_plan_baselinesfunction. Compare performance before and after using your query parameter bindings. If the new plan is fast, you can add it to your approved plans. If it's faster than another plan for the same parameter bindings, then you can mark the slower plan as Rejected.For more information, see Approving better plans. For reference information about this function, see apg_plan_mgmt.evolve_plan_baselines.
For more information, see Ensuring
consistent performance after major version upgrades with Amazon Aurora PostgreSQL-Compatible Edition Query Plan Management
Note
When you perform a major version upgrade using logical replication or AWS DMS, make sure that you replicate the apg_plan_mgmt schema to ensure existing plans are copied to the upgraded instance.
For more information on logical replication, see Using logical replication to perform a major version upgrade for Aurora PostgreSQL.
Reactive plan management to detect and repair performance regressions
By monitoring your application as it runs, you can detect plans that cause performance regressions. When you detect regressions, you manually reject or fix the bad plans by following these steps:
While your application runs, enforce the use of managed plans and automatically add newly discovered plans as unapproved. For more information, see Using Aurora PostgreSQL managed plans and Automatically capturing plans.
Monitor your running application for performance regressions.
When you discover a plan regression, set the plan's status to
rejected. The next time the optimizer runs the SQL statement, it automatically ignores the rejected plan and uses a different approved plan instead. For more information, see Rejecting or disabling slower plans.In some cases, you might prefer to fix a bad plan rather than reject, disable, or delete it. Use the
pg_hint_planextension to experiment with improving a plan. Withpg_hint_plan, you use special comments to tell the optimizer to override how it normally creates a plan. For more information, see Fixing plans using pg_hint_plan.
