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
Data Manipulation (SQL/Python)
Data Scientist
Vercel
October 5, 2025Data Scientist
Take-home Project
Data Manipulation (SQL/Python)
Easy
105
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
JOIN types and when to use each
NULL handling and COALESCE
Data cleaning and transformation
Index optimization and query performance
Aggregate functions and GROUP BY
Pandas vectorized operations and groupby
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?
Sharpen Your Skills on Codemia
Practice similar problems with our interactive workspace, get AI feedback, and track your progress.
Practice SQL ProblemsSample Answer
Problem Understanding
The query involves an events table that logs various activities. The primary goal is to identify and optimize a slow query that aggregates event data based on specific criteria. The query should pro...
Approach
- Review the Current Query: Begin by analyzing the existing SQL query to identify inefficiencies, such as missing indexes or unnecessary joins.
- Identify Critical Columns: Focus on the colu...
Submit Your Answer
Markdown supported