🔗 The DISTINCT in your COUNT

The query SELECT count(DISTINCT user_id) FROM events; might seem efficient, but it can lead to performance pitfalls. The keyword DISTINCT disables parallel query execution in PostgreSQL, forcing the database to process all data serially. This occurs because the aggregate COUNT(DISTINCT ...) requires a unique count, which cannot use parallel processing effectively due to the need for a specific order. Without parallel execution, queries on large tables become inefficient, often resulting in costly disk usage and longer processing times. The article offers a solution: rewriting the query to use a GROUP BY clause instead. This approach allows for parallel processing, significantly improving query execution time with larger datasets while still achieving the desired unique count result. This insight is crucial for developers optimizing analytics queries for better performance.