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Describe a time when you had to optimize a slow SQL query. What steps did you take?

SQLmedium0.5 min read

**Situation**: A critical exec report was timing out at 30+ minutes; SLA was 5 minutes. **Task**: Diagnose and fix without changing business logic. **Action**: I ran EXPLAIN (ANALYZE) and found: (1) missing index on join key causing full table scan, (2) cross join that could be...

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Frequency
Low
Asked at 4 companies
Category
487
questions in SQL
Difficulty Split
130E|271M|86H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
AareteAccentureFossil GroupYash Technologies
Interview Pro Tip

Red Flag: Saying 'I added an index' without explaining how you identified the bottleneck. Pro-Move: Mention EXPLAIN ANALYZE, predicate pushdown, and early filtering—shows systematic debugging.

Key Concepts Tested
joinsql

Why This Question Matters

This medium-level SQL question appears frequently in data engineering interviews at companies like Aarete, Accenture, Fossil Group, and 1 others. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (join, sql) 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
108 words

Situation: A critical exec report was timing out at 30+ minutes; SLA was 5 minutes. Task: Diagnose and fix without changing business logic. Action: I ran EXPLAIN (ANALYZE) and found: (1) missing index on join key causing full table scan, (2) cross join that could be inner join with a filter, (3) filter in HAVING that could move to WHERE to reduce rows early, (4) unnecessary ORDER BY on a subquery. I added the index, rewrote the join, pushed predicates down, and removed redundant sort. Result: Runtime dropped to under 2 minutes; SLA met. I documented the changes and set up a query review process for similar reports.

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