Why Doesn't My Oracle 11g Query Parallelize?

Why Oracle 11g Doesn't Force Parallelization for Single SQL Queries

Understanding Oracle parallelism's complexities is crucial. To optimize parallelism effectively, it requires a deep understanding of Oracle, meticulous examination of parameters, testing prolonged queries, and a healthy dose of skepticism.

Determining Parallel Execution

When troubleshooting parallelism issues, three key questions arise:

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

Using SQL Monitoring

To answer these questions efficiently, utilize SQL Monitoring with active reports. Use the following query to generate an HTML report:

SELECT DBMS_SQLTUNE.REPORT_SQL_MONITOR(SQL_ID => 'YOUR_SQL_ID', TYPE => 'ACTIVE') FROM DUAL;

This report provides detailed information on how much time was spent on each step of the execution plan, including the extent of parallelism utilized.

Evaluating Explain Plan Output

The EXPLAIN PLAN output also provides valuable insights. The Notes section may reveal why the query did not request parallelism. Check for notes indicating limitations or hints that could hinder parallelization.

Understanding Factors Influencing Parallel Server Allocation

The number of parallel servers allocated is influenced by a myriad of factors, some of the most significant being:

• Query hint: Use /* PARALLEL */ or /* PARALLEL(DOP) */ hints to force parallelism.
• Inter-operation parallelism: Sorting or grouping operations can double the number of parallel servers allocated.
• Recursive SQL: Recursive SQL operations can serialize parallel execution.
• Table and index degree: Larger degree values may increase parallelism.

Other Relevant Configuration Parameters

Beyond hints and execution plan analysis, consider these system parameters that can impact parallelism:

• PARALLEL_DEGREE_LIMIT and PARALLEL_DEGREE_POLICY define limits and policies for parallel degree allocation.
• PARALLEL_AUTOMATIC_TUNING automatically adjusts parallelism based on system load.
• PARALLEL_THREADS_PER_CPU and CPU_COUNT influence the number of parallel servers allocated per CPU.

Conclusion

Oracle parallelism is a powerful tool, but its optimization requires careful consideration of numerous factors and a deep understanding of Oracle's execution engine. By leveraging available tools, examining system parameters, and addressing specific execution plan issues, you can effectively utilize parallelism in Oracle 11g to enhance query performance.

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