What causes PostgreSQL to choose sequential scan over index?

Poor selectivity—when your WHERE grabs most rows, random I/O jumps cost more than steady seq read. Tune stats with ANALYZE.

How do I force PostgreSQL to use an index?

Don't—it's usually wrong. But disable_seqscan=on for testing, or use partial indexes for selective cases.

Nope. Depends on costs. Benchmark with EXPLAIN ANALYZE; partition if seq dominates huge ranges.

☁️ Cloud & Databases

You build the perfect index. Run the query. Crickets. PostgreSQL just seq-scanned your million-row table anyway. Here's why—and how to fight back.

Open Source Beat Apr 11, 2026 4 min read

PostgreSQL picks plans by total cost, not index presence—seq scans win on low-selectivity queries. 𝕏
Tune with ANALYZE, partial indexes, and partitioning to make indexes viable. 𝕏
Think like Postgres: minimize I/O work, ignore the hype around 'always index.' 𝕏

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#EXPLAIN ANALYZE #PostgreSQL #database indexes #query-optimization

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Originally reported by Dev.to