Optimize a slow query on transactions
Last updated: December 25, 2025
Quick Overview
A query on transactions is running slowly. Identify the bottleneck and optimize it.
Adobe
Data Manipulation (SQL/Python)
Data Scientist
Adobe
December 25, 2025Data Scientist
Onsite
Data Manipulation (SQL/Python)
Easy
7
10
3,711 solved
A query on transactions is running slowly. Identify the bottleneck and optimize it.
Adobe asks this during the Onsite because data engineering skills are critical for the role. You should be comfortable with complex joins, window functions, CTEs, and performance optimization.
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
Aggregate functions and GROUP BY
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
Index optimization and query performance
Subqueries and correlated subqueries
Date/time manipulation
JOIN types and when to use each
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?
- How would you handle slowly changing dimensions in this scenario?
- How would you handle this if the data was spread across multiple databases?
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Practice SQL ProblemsSample Answer
Problem Understanding
In this scenario, we are dealing with a transactions table that likely contains columns such as transaction_id, user_id, amount, transaction_date, and possibly others. The goal is to optimize ...
Approach
- Identify the Current Query: Start by examining the existing slow query to understand its structure and identify potential bottlenecks.
- Analyze Execution Plan: Use
EXPLAINto check th...
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