index not used with cte
Problem
Index (ivfflat at least) are not used when the query is a cte : [code block] ==> [code block] Whereas the index is used using a subquery : [code block] [code block] Test on 0.80.0 pgvector and postgreSQL 15 (windows) Test on 0.80.0 pgvector and postgreSQL 17 (linux/docker)
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1 Fix
Optimize CTE Queries to Utilize Indexes
Commonly, PostgreSQL's query planner may not utilize indexes when dealing with Common Table Expressions (CTEs) due to the way CTEs are treated as optimization fences. This means that the planner may not consider the indexes available for the underlying tables when executing the CTE, leading to suboptimal query performance. In contrast, subqueries may allow the planner to better optimize the use of indexes.
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- 1
Rewrite CTE as Subquery
Convert the CTE into a subquery to allow PostgreSQL to optimize the query plan and utilize the available indexes effectively.
sqlSELECT * FROM (SELECT ... FROM your_table WHERE ...) AS subquery; - 2
Analyze Query Performance
Use the EXPLAIN command to analyze the performance of the rewritten query. This will help confirm that the index is being utilized as expected.
sqlEXPLAIN ANALYZE SELECT * FROM (SELECT ... FROM your_table WHERE ...) AS subquery; - 3
Check Index Usage
Verify that the index (ivfflat) is being used in the execution plan. Look for 'Index Scan' in the output of the EXPLAIN command.
sqlEXPLAIN SELECT * FROM your_table WHERE your_vector_column <-> your_vector_value < threshold; - 4
Test Across Environments
Run the optimized query on both PostgreSQL 15 and 17 environments to ensure consistent performance improvements and index usage across different setups.
sqlSELECT * FROM (SELECT ... FROM your_table WHERE ...) AS subquery; - 5
Monitor Performance Metrics
After implementing the changes, monitor the performance metrics of the database to ensure that query execution times have improved and that the index is being utilized effectively.
sqlSELECT * FROM pg_stat_statements WHERE query LIKE '%your_table%';
Validation
Confirm that the rewritten query shows improved performance and that the index is used by checking the execution plan with EXPLAIN. Additionally, compare execution times before and after the changes to ensure that the fix has had the desired effect.
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Environment
Submitted by
Alex Chen
2450 rep