effective_cache_size

effective_cache_size

  • Understanding effective_cache_size: The effective_cache_size parameter in PostgreSQL is an estimator used by the query planner to determine the effectiveness of the system's cache, which includes both the PostgreSQL shared_buffers and the operating system's cache. It helps the planner make more informed decisions about query execution strategies, particularly in choosing between index scans and sequential scans. Setting this parameter does not allocate memory but informs the planner about the memory available for caching. An accurately set effective_cache_size can lead to more efficient query plans, while a misconfiguration might result in suboptimal query performance.
  • Tuning Recommendations:
    • Reflect System Resources: Set effective_cache_size to a value close to, or slightly less than, the total amount of memory available for caching by PostgreSQL and the operating system combined. For systems with dedicated PostgreSQL usage, this could be up to 75% of the total system memory. The goal is to give the planner a realistic estimate of how much data can be cached, enhancing its ability to choose the most efficient execution plans.
    • Dynamic Adjustment: The optimal setting for effective_cache_size can change as the workload and data volume evolve. Regular monitoring of query performance and execution plans can indicate when adjustments are needed. Use tools like EXPLAIN to analyze query plans and adjust the effective_cache_size based on whether the planner's choices align with the actual performance outcomes.
  • Balancing Accuracy and Performance: An accurate setting for effective_cache_size helps the query planner optimize the use of available memory resources without directly affecting memory allocation. This setting should be balanced with other memory configuration parameters and the memory needs of other applications on the server. It's important to consider the overall system usage pattern, including peak load times and the nature of the queries being run. Adjustments to effective_cache_size should be made cautiously, with careful observation of their impact on query planning and execution performance. Remember, while effective_cache_size is a hint to the planner, actual runtime performance should be the ultimate guide to tuning this parameter.