Discover why the `parallel hint` is not functioning properly for views in Oracle 19c and explore effective solutions and considerations for performance optimization.
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Understanding the Parallel Hint Issue in Oracle 19c
When migrating code from Oracle 12c to Oracle 19c, many developers encounter unexpected challenges, such as issues with the parallel hint not working for views. This can be a perplexing problem, especially when the code that worked in the earlier version fails in the new environment. Let's delve into this issue and explore its possible resolutions.
The Problem
During the migration process, you may notice that while using the following code snippet:
[[See Video to Reveal this Text or Code Snippet]]
The parallel degree appears to be functioning correctly. However, when you explicitly specify the view’s column names:
[[See Video to Reveal this Text or Code Snippet]]
The parallel hint seems to be ineffective in Oracle 19c, contrary to its successful operation in Oracle 12c. This inconsistency raises concerns about how to resolve this issue effectively and ensure optimal performance.
Explanation of the Behavior
Upon examination, it is essential to understand that the behavior you are observing is likely due to how Oracle manages parallel processing and execution plans in different versions. Here's a breakdown of the reasoning:
Execution Plan Changes:
With each new version, Oracle may alter its execution plans based on numerous factors, including the data structure and statistics gathered. These changes do not necessarily indicate that the database is malfunctioning, but instead reflect its optimized way of handling parallelism based on current data and requests.
Parallel Processing Logic:
When the query aggregates results to a single row (like using COUNT), subsequent operations may not benefit from parallel processing. For example, once a block has reduced the result set to one row, parallel executions across multiple threads become redundant, as there is no additional data to process concurrently.
Effective Use of Parallelism:
Oracle will utilize parallelism where it deems efficient. If the amount of data is small (in your case, a table containing only 13K rows), Oracle may choose not to use parallel processing because the overhead of setting up parallel execution outweighs any potential benefits.
Recommended Solutions
Given these insights, you may take the following steps to address the parallel hint issue in your Oracle 19c environment:
1. Evaluate the Need for Parallelism
Remove the Hint: Sometimes, less is more. If your query can run efficiently without the parallel hint, consider executing it serially.
Assess Query Complexity: Consider whether your queries are complex enough to benefit from parallel execution. If they're not performing resource-intensive operations (like large joins or sorts), parallel hints may not yield considerable performance improvements.
2. Review Execution Plans
Examine Your Execution Plans: Continuously analyze the execution plans generated by Oracle to identify how it processes your queries. If execution times are acceptable, the way Oracle decides to execute your queries should not be a cause for concern.
3. Optimize Data Management
Statistics Maintenance: Ensure that your database statistics are up to date. The optimizer relies on these statistics to make informed decisions on query execution plans.
Consider Partitioning: If applicable, partitioning large tables may help improve performance for certain operations, thus leveraging parallel processing where appropriate.
Conclusion
In summary, the parallel hint issue encountered during the transition from Oracle 12c to 19c is primarily rooted in the changes in execution plans and the new version's optimization strategies. Understanding the context of your queries and the data being processed can help alleviate concerns surrounding performance. A
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