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Given an unoptimized query execution plan, how would you diagnose and improve performance?

SQLmedium0.3 min readPremium
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
BCG
Key Concepts Tested
joinpartition
Expert AnswerPremium
55 wordsInterview-ready
**Diagnose**: (1) EXPLAIN ANALYZE—actual vs. estimated rows; large gap = stale stats. (2) Identify: Seq Scan on large table, nested loop with high cost, sort spill. (3) Check join order—small table should drive when possible. **Improve**: Add index on filter/join columns; ANALYZE; rewrite (join vs. subquery); partition prune; materialized view....
The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations. It covers performance optimization strategies, common pitfalls to avoid, and real-world examples from companies like BCG. The answer also includes follow-up discussion points that interviewers commonly explore.

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