Optimize a slow query on users
Last updated: December 31, 2025
Quick Overview
A query on users is running slowly. Identify the bottleneck and optimize it.
Compass
Data Manipulation (SQL/Python)
Data Scientist
Compass
December 31, 2025Data Scientist
Onsite
Data Manipulation (SQL/Python)
Medium
15
5
1,537 solved
A query on users is running slowly. Identify the bottleneck and optimize it.
This question from Compass'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
Subqueries and correlated subqueries
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
Index optimization and query performance
Data cleaning and transformation
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
- How would you validate the correctness of your query results?
- What would you do if this query needs to run every 5 minutes?
- Can you rewrite this without using subqueries?
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 a users table that contains user data such as user_id, join_date, and last_active_date. The goal is to optimize a slow query that likely retrieves user information based on ...
Approach
To optimize the query, follow these steps:
- Analyze the current query: Identify the parts of the query that are causing slow performance, such as correlated subqueries or multiple joins.
- **Us...
Submit Your Answer
Markdown supported