- 09 Dec, 2025
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- 2 Mins Read
10 Hidden Oracle Parameters That Boost Performance
10 Hidden Oracle Parameters That Boost Performance
Introduction
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Briefly explain why Oracle performance tuning is critical.
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Introduce the concept of hidden/underscore (“_”) parameters.
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Disclaimer: hidden parameters should be changed only with Oracle Support guidance.
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Tease that the blog covers 10 highly impactful hidden parameters.
What Are Hidden Oracle Parameters?
Why Oracle Uses Underscore Parameters
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Internal engine controls
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Optimizer behavior, memory, caching
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Not visible in standard parameter lists
Risks of Using Hidden Parameters
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Support restrictions
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Upgrade conflicts
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Instability if misconfigured
How to Check Hidden Parameters in Oracle
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Querying
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Query examples
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Caution before modifying
Top 10 Hidden Oracle Parameters That Improve Performance
1. _optimizer_use_feedback
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Improves cardinality and plan stability
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Helps when statistics fluctuate
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When to use / when to avoid
2. _serial_direct_read
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Enables direct reads for full table scans
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Boosts performance for large tables
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Ideal for data warehouse environments
3. _small_table_threshold
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Controls when table is considered “small”
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Impacts caching behavior
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Best use cases during OLTP bursts
4. _optimizer_max_permutations
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Reduces excessive optimizer permutations
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Useful in systems with complex joins
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Prevents CPU spikes during parsing
5. _db_file_optimizer_read_count
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Influences multiblock I/O behavior
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Helps in tuning large sequential reads
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Can improve throughput
6. _cursor_obsolete_threshold
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Controls cursor aging
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Boosts performance in high-soft-parse environments
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Helps shared pool efficiency
7. _sqltune_category
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Enables custom tuning category
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Useful when using SQL Tuning Advisor
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Separates business workloads
8. _undo_autotune
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Improves undo behavior
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Prevents ORA-01555 in heavy workloads
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Optimizes retention automatically
9. _optimizer_cost_model
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Chooses between CPU-based and IO-based cost model
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Impacts execution plans
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Better performance in certain workloads
10. _row_prefetch_enabled
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Improves fetch efficiency for sequential row access
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Helpful in PL/SQL loops
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Boosts response time for batch operations
Real-Time Examples of Performance Improvement
Example 1: Fixing Slow Full Table Scans
Example 2: Stabilizing Query Plans
Example 3: Reducing Parse Time
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Before/after results
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Statistics comparison
Best Practices Before Changing Hidden Parameters
Document Every Change
Test in Lower Environments First
Follow Oracle Support Guidelines
Use Baseline Plans to Avoid Regression
Conclusion
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Hidden parameters are powerful but risky
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When used correctly, they offer significant performance boosts
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Always tune using AWR, ASH, and SQL Monitor before touching hidden parameters
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