Mastering SQL Performance with SQL Plan Management

  In my previous blogpost, we delved into the complete details on loading a SQL Plan with its Plan Hash Value.  SQL Plan Management (SPM) is a powerful feature designed to ensure consistent and optimized SQL performance by managing execution plans proactively. By leveraging SQL Plan Baselines, SPM helps maintain query efficiency and reliability, making it an essential tool for database administrators. Here's a closer look at how SPM works, its benefits, and its role in maintaining SQL performance.

What is SQL Plan Management?

SQL Plan Management is a preventive mechanism that allows the database optimizer to manage execution plans by using a set of accepted plans, known as SQL Plan Baselines. These baselines ensure that only verified plans are used, preventing unexpected performance degradation.

Benefits of SQL Plan Management

• Proactive Performance Protection: SPM helps avert performance issues that could arise from changes in SQL execution plans. By using a set of verified plans, it mitigates risks associated with plan changes.

• Adaptation to Changes: The feature gracefully adapts to changes by verifying and accepting only those plan changes that improve performance. This ensures that the optimizer is always using the most effective execution plans.

• Minimized Impact from Upgrades: Post-database upgrade, SQL plans may change due to alterations in optimizer versions. SQL Plan Baselines minimize the risk of performance issues from such changes, ensuring a smoother transition.

• Stabilized SQL Performance: System, data, and database modifications can impact SQL plans and lead to performance issues. SPM helps stabilize SQL performance by minimizing regressions and maintaining consistent query execution.

SQL Baselines vs. SQL Profiles

While both SQL Plan Baselines and SQL Profiles aim to optimize SQL performance, they differ in their approach and application:

Proactive vs. Reactive: SQL Plan Baselines are proactive, capturing and enforcing specific execution plans. SQL Profiles, on the other hand, are reactive, adjusting optimizer cost estimates based on observed performance.

Plan Reproduction vs. Cost Correction: SQL Plan Baselines reproduce specific execution plans for SQL statements, ensuring consistency. SQL Profiles correct the optimizer’s cost estimates to improve query performance but do not enforce specific plans.

Execution Frequency: SQL Plan Baselines are only created for SQL statements that have been executed more than once. Statements executed fewer times are not considered for baselining.

SQL Plan Evaluation

The process of SQL Plan Evaluation involves the optimizer verifying new plans and adding them to existing SQL Plan Baselines. Typically, a SQL Plan Baseline for a statement starts with one accepted plan, which is then evaluated and refined over time.

SQL Management Base (SMB)

The SQL Management Base (SMB) is a logical repository within the data dictionary that houses SQL profiles, SQL patches, SQL Plan history, and SQL statement logs (containing SQL IDs). Located in the SYSAUX tablespace, SMB utilizes Automatic Segment-Space Management. For Pluggable Databases (PDBs), SMB data is stored within the PDB and is included if the PDB is unplugged.

Conclusion

SQL Plan Management is a vital tool for maintaining optimal SQL performance and ensuring database stability. By using SQL Plan Baselines to manage and enforce execution plans, it helps prevent performance issues, adapt to changes, and handle upgrades smoothly. Understanding the distinctions between SQL Baselines and SQL Profiles, along with the role of the SQL Management Base, can further enhance your ability to optimize and manage SQL performance effectively.