PostgreSQL Query Optimizer: Troubleshooting Unconsidered Hints

PostgreSQL’s query optimizer (or planner) creates optimal execution plans for SQL queries by evaluating multiple possible plans and selecting the fastest one. [^1]

Understanding PostgreSQL’s Approach to Optimizer Hints

PostgreSQL takes a deliberately cautious approach to implementing optimizer hints, based on several key concerns:

1. Scaling issues: Hints that perform well with small datasets often lose effectiveness as tables grow. [^2]
2. Unnecessary overrides: The PostgreSQL optimizer typically makes optimal decisions without hints. [^2]
3. Impedes optimizer improvements: When users rely on hints, they’re less likely to report query planning issues to the PostgreSQL project, which slows optimizer development. [^2]

Troubleshooting When Hints Aren’t Considered

If you’re using hints (typically through an extension like pg_hint_plan) and they’re not being considered, follow these steps:

1. Verify Your Query Plan

Use the EXPLAIN command to examine the execution plan:

EXPLAIN (ANALYZE, BUFFERS) YOUR_QUERY;

While understanding query plans takes experience, it’s crucial for diagnosing optimizer issues. [^3]

2. Check Extension Compatibility

When using pg_hint_plan or another hint extension, verify:

• The extension is properly installed
• You’re using the correct syntax for your PostgreSQL version
• The extension is compatible with your PostgreSQL version [^4][^5]

3. Consider Query Complexity

For complex queries with multiple table joins, PostgreSQL may use the genetic query optimizer (GEQO), which employs heuristic searching instead of exhaustive planning. This can cause certain hints to be ignored. [^6][^7]

4. Review Parallel Query Settings

For parallel execution hints:

• Check that max_parallel_workers and max_parallel_workers_per_gather are properly configured
• Note that parallel hints behave differently in SELECT queries versus DML operations [^5][^8]

5. Alternative Approaches

Instead of hints, consider these alternatives:

• Restructuring your query
• Using Common Table Expressions (CTEs) with the MATERIALIZED option to control execution flow [^9]
• Adjusting statistics with ALTER TABLE SET STATISTICS
• Reviewing and updating table statistics with ANALYZE

Performance Tuning Without Hints

PostgreSQL provides several effective optimization methods without using explicit hints:

1. Proper indexing: Create appropriate indexes for your query patterns
2. Up-to-date statistics: Run ANALYZE regularly
3. Query restructuring: Rewrite problematic queries
4. Configuration tuning: Adjust PostgreSQL configuration parameters for your workload [^10][^11]

Keep in mind that PostgreSQL’s optimizer makes decisions based on statistics and cost estimates. Before resorting to hints, ensure your database statistics are current and your schema design is sound.

References

[^1]: [PostgreSQL: Documentation: 17: 50.5. Planner/Optimizer](https://www.postgresql.org/docs/current/planner-optimizer.html#:~:text=A given,the fastest.) 

[^2]: [OptimizerHintsDiscussion – PostgreSQL wiki](https://wiki.postgresql.org/wiki/OptimizerHintsDiscussion#:~:text=Does not,the project.) 

[^3]: [Documentation: 17: 14.1. Using EXPLAIN](https://www.postgresql.org/docs/current/using-explain.html#:~:text=You can,to master.) 

[^4]: Re: Parallel hints in PostgreSQL with consistent perfromance 

[^5]: PostgreSQL: Parallel hints with pg_hint_plan extension not working with … 

[^6]: [PostgreSQL: Documentation: 17: 60.1. Query Handling as a Complex…](https://www.postgresql.org/docs/current/geqo-intro.html#:~:text=This makes,of tables.) 

[^7]: [Documentation: 17: 19.7. Query Planning](https://www.postgresql.org/docs/current/runtime-config-query.html#:~:text=The genetic,complex queries.) 

[^8]: [Re: Parallel hints in PostgreSQL with consistent perfromance](https://www.postgresql.org/message-id/5ecd893b-be4c-72ea-4896-e0f9afcf33d3%40enterprisedb.com#:~:text=the degree,of max_parallel_workers_per_gather.) 

[^9]: 17: 7.8. WITH Queries (Common Table Expressions) 

[^10]: [PostgreSQL: Documentation: 17: Chapter 14. Performance Tips](https://www.postgresql.org/docs/current/performance-tips.html#:~:text=Query performance,the system.&text=This chapter,Submit correction.) [

^11]: Documentation: 8.0: Performance Tips