Why PostgreSQL Ignores Index & Uses Sequential Scan | Real Examples + Fixes
Welcome back to Dev Portal.
Ever wondered why PostgreSQL chooses a Sequential Scan even when you've created an index? You're not alone! In this video, we break down:
What you'll learn:
What is a Sequential Scan?
When PostgreSQL prefers Index Scan vs. Seq Scan
Top reasons PostgreSQL ignores your index
Real-world EXPLAIN ANALYZE output on 50 million rows
How to force index usage (and why you probably shouldn’t)
Best practices to help the planner pick the right plan
Important Links
Notes: https://github.com/nakulmitra/databas...
PostgreSQL Documentation – Query Planning: https://www.postgresql.org/docs/curre...
PostgreSQL Indexing Strategies: https://www.postgresql.org/docs/curre...
Timestamp:
00:00 - Introduction
00:51 - What is a Sequential Scan?
01:14 - When Does PostgreSQL Use an Index Scan?
04:12 - Why Does PostgreSQL Ignore the Index Sometimes? (Low Selectivity, Table Is Small, Missing Statistics, Parallel Execution, Function Usage or Type Mismatch)
14:22 - Real-World Example (Your Case)
21:40 - Forcing Index Usage (Only for Testing)
22:59 - Summary
Real-world use case: I demonstrate a query over 50 million records, analyze the query plan, and explain why PostgreSQL still chooses a Parallel Seq Scan - even with an index.
Fixes Covered:
Creating functional indexes
Running ANALYZE
Understanding selectivity and planner decisions
Takeaway
PostgreSQL does not guarantee index usage. Instead, it chooses the most efficient path for execution based on the data size and distribution. If a large portion of the table needs to be read, sequential scan is usually faster - even if an index exists.
Whether you're optimizing your app’s performance or just learning PostgreSQL internals, this is a must-watch!
Check out my previous tutorial on VACUUM & ANALYZE for better index usage: • PostgreSQL Performance Optimization: Remov...
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