Why Isn't My Oracle 11g Query Using Multiple CPUs?

Exploring Oracle 11g Parallel Query Limits

Regarding your issue of Oracle 11g not utilizing multiple CPUs for a specific SQL query, it's crucial to comprehend the intricacies of Oracle parallelism. Optimizing this requires substantial knowledge and careful analysis of various factors.

Proper Inquiry

When troubleshooting parallelism, consider the following questions:

1. How many parallel servers were requested?
2. How many parallel servers were allocated?
3. How many parallel servers were effectively used?

Effective Tools

• SQL Monitoring with Active Reports: Provides detailed information on each execution step, revealing parallelism usage and potential bottlenecks.
• explain plan for ... and select * from table(dbms_xplan.display);: Analyzes the query plan and includes notes explaining why parallelism may not have been applied.

Factors Influencing Parallelism

Numerous factors determine the number of parallel servers allocated, including:

• Inter-operation parallelism (e.g., sorting, grouping)
• Query hints
• Recursive SQL
• Alter session parameters
• Table and index degree
• Optimizer cost considerations
• Plan management mechanisms
• Oracle edition and licensing
• System parameters (e.g., PARALLEL_ADAPTIVE_MULTI_USER, PARALLEL_DEGREE_LIMIT)
• CPU count and system load
• Parallel DML restrictions
• Bugs and parsing issues

Troubleshooting Methods

To troubleshoot your specific query, try the following:

• Check the Explain Plan Notes: Look for any reasons why parallelism was not requested.
• Analyze SQL Monitoring Reports: Identify slow steps and parallelism utilization.
• Adjust System Parameters: Consider modifying parameters like PARALLEL_DEGREE_LIMIT and PARALLEL_MIN_SERVERS.
• Enable Parallel Hints: Explicitly request parallelism using statement-level or object-level hints.
• Exclude Non-Parallel Objects: Use object-level noparallel hints to force serial execution for specific tables.
• Review Parallel DML Restrictions: Ensure that your query does not violate any of the limitations on parallel DML.

Remember, parallelism is a nuanced feature and requires thorough investigation and understanding to optimize effectively.

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