Answer
Sequential scans occur when tables are small, statistics are outdated, or index selectivity is poor.
Quick Answer
Seq scan can be faster than index scan when: the query retrieves a large fraction of the table (reading index then fetching most pages is worse than just reading pages sequentially), the table fits in shared_buffers (already cached), or random_page_cost is set high (sequential I/O is cheaper). Lower random_page_cost (SSD: 1.1 vs HDD: 4.0) encourages PostgreSQL to use indexes more.
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SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ