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The Impact of ALTER SESSION SET OPTIMIZER_DYNAMIC_SAMPLING=0 on Oracle Database Performance
Oracle Database is a powerful tool used by organizations worldwide to manage and store their data efficiently. One key aspect of Oracle Database performance tuning is the optimizer dynamic sampling feature, which controls the level of statistics sampling performed by the query optimizer. In this article, we will explore the impact of setting OPTIMIZER_DYNAMIC_SAMPLING=0 at the session level and how it can affect query performance.
Understanding OPTIMIZER_DYNAMIC_SAMPLING
OPTIMIZER_DYNAMIC_SAMPLING is a parameter in Oracle Database that controls the level of statistics sampling performed by the query optimizer. When set to a value greater than 0, the optimizer dynamically samples data during query optimization to generate more accurate execution plans. This can be useful in scenarios where the data distribution is skewed or when the statistics are not up to date.
The Impact of Setting OPTIMIZER_DYNAMIC_SAMPLING=0
When OPTIMIZER_DYNAMIC_SAMPLING is set to 0 at the session level using the ALTER SESSION command, the query optimizer will no longer perform dynamic sampling during query optimization.
. This means that the optimizer will rely solely on the existing statistics to generate execution plans, which can lead to suboptimal performance in certain scenarios.
Case Study: Performance Degradation
Consider a scenario where a table has a skewed data distribution, but the statistics are not up to date. In this case, setting OPTIMIZER_DYNAMIC_SAMPLING=0 can result in the optimizer generating an execution plan based on inaccurate statistics, leading to poor query performance. This can manifest as slower query execution times, increased CPU and I/O usage, and overall degraded system performance.
Best Practices for Using OPTIMIZER_DYNAMIC_SAMPLING
- Regularly update statistics using the DBMS_STATS package to ensure accurate data distribution information.
- Use OPTIMIZER_DYNAMIC_SAMPLING sparingly and only in scenarios where dynamic sampling is necessary to improve query performance.
- Monitor query performance using tools like Oracle Enterprise Manager to identify and address performance issues proactively.
Conclusion
In conclusion, setting OPTIMIZER_DYNAMIC_SAMPLING=0 at the session level can have a significant impact on Oracle Database performance. It is important to carefully consider when and how to use this parameter to ensure optimal query performance. By following best practices and monitoring query performance, organizations can leverage the power of Oracle Database while maintaining high performance levels.
For more information on Oracle Database performance tuning, check out Oracle’s official documentation.
