Optimize a slow query on events
Last updated: October 5, 2025
Quick Overview
A query on events is running slowly. Identify the bottleneck and optimize it.
Vercel
October 5, 2025105
8
3,109 solved
A query on events is running slowly. Identify the bottleneck and optimize it.
This question from Vercel's Take-home Project 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
- Write syntactically correct SQL with proper JOIN and WHERE clauses
- Use GROUP BY and aggregate functions appropriately
- Handle NULL values correctly in your queries
- Explain the query execution plan at a high level
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
- What would you do if this query needs to run every 5 minutes?
- Can you rewrite this without using subqueries?
- How would you handle slowly changing dimensions in this scenario?
- How would you optimize this query for a table with 100 million rows?
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