Optimize a slow query on sessions
Last updated: February 20, 2026
Quick Overview
A query on sessions is running slowly. Identify the bottleneck and optimize it.
Shopify
Data Manipulation (SQL/Python)
Data Scientist
Shopify
February 20, 2026Data Scientist
Onsite
Data Manipulation (SQL/Python)
Hard
31
15
4,559 solved
A query on sessions is running slowly. Identify the bottleneck and optimize it.
Data manipulation questions at Shopify test your ability to work with real-world datasets. This Onsite question evaluates your SQL proficiency, understanding of data modeling, and ability to derive insights from raw data.
What the Interviewer Expects
- Solve complex analytical problems with elegant, readable SQL
- Optimize queries for large-scale datasets with partitioning and indexing
- Use recursive CTEs, lateral joins, and advanced window functions
- Design the data model alongside the query solution
- Discuss trade-offs between SQL and programmatic approaches (Python/pandas)
- Consider the operational aspects: query scheduling, incremental processing
Key Topics to Cover
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
NULL handling and COALESCE
JOIN types and when to use each
Date/time manipulation
Common Table Expressions (CTEs)
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 handle this if the data was spread across multiple databases?
- What indexes would you create to support this query?
- What would you do if this query needs to run every 5 minutes?
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Practice SQL ProblemsSample Answer
Problem Understanding
The query involves a dataset of user sessions, which typically includes fields such as session_id, user_id, start_time, end_time, and possibly other metrics related to user interactions. The g...
Approach
- Identify Requirements: Clearly define what information is needed from the sessions table. For example, we might want the user ID, session duration, and the rank of sessions by duration.
- **An...
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