Understanding SQL Query Result Cache

The SQL Query Result Cache is a feature that improves query performance by storing query results in memory. When enabled, subsequent executions of the same query can retrieve the cached result, avoiding redundant computation or disk reads. Let’s explore its capabilities, rules, and limitations.

Key Facts About SQL Query Result Cache

1. Configuration Levels:
   • The cache can be enabled or disabled at the system, session, or query level.
   • Example:

     ALTER SYSTEM SET RESULT_CACHE_MODE = FORCE; -- At system level
     ALTER SESSION SET RESULT_CACHE_MODE = FORCE; -- At session level
     SELECT /*+ RESULT_CACHE */ * FROM employees; -- At query level
     

2. Invalidation of Cached Results:
   • Cached results become invalid when the underlying data is modified by DML (INSERT, UPDATE, DELETE) operations.
3. Conditions for PL/SQL Function Caching:
   • The function must not be defined in an anonymous block.
   • The function must have at least one OUT or IN OUT parameter.

Query Types That Cannot Be Cached

Certain queries cannot utilize the SQL Query Result Cache due to their dynamic or non-deterministic nature:

1. Subquery Results:
   • Results of subqueries are not cached.
2. Queries Using Non-Deterministic Functions:
   • Queries with functions like SYSDATE or SYS_TIMESTAMP are excluded from caching.
3. Queries on Dictionary or Temporary Tables:
   • Caching is not applied to queries involving dictionary or temporary tables.
4. Dynamic SQL:
   • Results involving dynamically constructed SQL statements are not cached.

Caching for PL/SQL Functions

When a PL/SQL Function’s Result Is Cached:

• If it is invoked with a parameter value for the first time in the session.
• When RELIES_ON clause specifies tables or views the function depends on, and no DML operation has invalidated the cache.

Example:

CREATE OR REPLACE FUNCTION get_employee_name (p_id NUMBER) 
RETURN VARCHAR2 
RESULT_CACHE RELIES_ON (employees) IS
BEGIN
    RETURN (SELECT first_name || ' ' || last_name 
            FROM employees 
            WHERE employee_id = p_id);
END;
/

When Results Are Invalidated:

• A session executes DML on tables or views specified in the RELIES_ON clause.

Benefits of SQL Query Result Cache

1. Performance Improvement:
   • Reduces database load by reusing cached results.
2. Memory Optimization:
   • Stores results in shared memory, accessible across sessions.
3. Flexibility:
   • Can be selectively enabled or disabled for queries or functions.

Limitations of SQL Query Result Cache

Best Practices for Using Result Cache

1. Cache Deterministic Queries:
   • Use result cache for static or rarely updated tables.
2. Avoid Non-Deterministic Functions:
   • Exclude functions like SYSDATE and DBMS_RANDOM.
3. Combine With RELIES_ON Clause:
   • Explicitly define dependencies for cached PL/SQL functions.

Conclusion

The SQL Query Result Cache is a powerful tool for optimizing query performance, but its effectiveness depends on understanding its limitations and using it appropriately. By caching deterministic queries and following best practices, you can significantly enhance database efficiency.

Would you like to dive deeper into examples or advanced use cases of SQL Query Result Cache? 😊

Ahmad Gohar

With over 18 years of experience in software architecture, Java technologies, and leadership, I specialize in crafting scalable, future-proof solutions for global organizations. Whether it’s transforming legacy systems, building cutting-edge cloud-native applications, or mentoring teams to excel, I’m committed to delivering value-driven results.

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