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Home/Questions/SQL/Explain the process you would follow for optimizing a database query that is running slow.

Explain the process you would follow for optimizing a database query that is running slow.

SQLmedium0.5 min read

**Systematic approach**: (1) **Profile**: EXPLAIN ANALYZE—identify seq scans on large tables, costly hash joins, sort spills. (2) **Baseline**: Current runtime, row counts, data distribution. (3) **Index**: Add B-tree/hash on filter and join columns; verify usage in plan. (4)...

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Frequency
Low
Asked at 1 company
Category
487
questions in SQL
Difficulty Split
130E|271M|86H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
Amazon
Key Concepts Tested
joinpartitionwindow

Why This Question Matters

This medium-level SQL question appears frequently in data engineering interviews at companies like Amazon. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (join, partition, window) will help you answer variations of this question confidently.

How to Approach This

Break this problem into components. Identify the core trade-offs involved, then walk the interviewer through your reasoning step by step. Demonstrate awareness of edge cases and production considerations - this is what separates good answers from great ones.

Expert Answer
90 words

Systematic approach: (1) Profile: EXPLAIN ANALYZE—identify seq scans on large tables, costly hash joins, sort spills. (2) Baseline: Current runtime, row counts, data distribution. (3) Index: Add B-tree/hash on filter and join columns; verify usage in plan. (4) Statistics: Run ANALYZE; planner accuracy affects join order. (5) Rewrite: Replace correlated subqueries with JOINs or window functions; push predicates. (6) Partition: Prune by partition key. (7) Tune: Work_mem, parallel workers. Scalability: At billion-row scale, missing partition filter can scan 100× more data. Cost: Index maintenance trade-off; materialized views for repeated aggregates.

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