Optimize a slow query on users
Last updated: October 10, 2025
Quick Overview
A query on users is running slowly. Identify the bottleneck and optimize it.
Redfin
October 10, 202545
4
1,586 solved
A query on users is running slowly. Identify the bottleneck and optimize it.
This question from Redfin's Onsite tests practical data skills. The interviewer wants to see clean, efficient queries that handle edge cases like NULLs, duplicates, and large datasets.
What the Interviewer Expects
- Use advanced SQL features: window functions, CTEs, subqueries
- Write efficient queries that avoid common performance pitfalls
- Handle complex data transformations with multiple joins and aggregations
- Discuss indexing strategy and query optimization
- Address data quality issues: duplicates, missing values, outliers
Key Topics to Cover
How to Approach This
- Clarify the schema and expected output format before writing queries.
- Use CTEs (WITH clauses) to break complex queries into readable steps.
- Consider window functions (ROW_NUMBER, RANK, LAG, LEAD) for ranking and sequential analysis.
- Watch for NULLs, duplicates, and edge cases in JOINs and GROUP BY.
- For pandas, prefer vectorized operations over row-by-row iteration.
Possible Follow-up Questions
- Can you rewrite this without using subqueries?
- How would you handle this if the data was spread across multiple databases?
- How would you optimize this query for a table with 100 million rows?
- What indexes would you create to support this query?
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