Optimize a slow query on transactions
Last updated: October 26, 2025
Quick Overview
A query on transactions is running slowly. Identify the bottleneck and optimize it.
Bloomberg
Data Manipulation (SQL/Python)
Data Scientist
Bloomberg
October 26, 2025Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Medium
30
5
967 solved
A query on transactions is running slowly. Identify the bottleneck and optimize it.
Bloomberg asks this during the Technical Screen 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
- 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
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
NULL handling and COALESCE
Common Table Expressions (CTEs)
Aggregate functions and GROUP BY
Subqueries and correlated subqueries
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 optimize this query for a table with 100 million rows?
- How would you handle slowly changing dimensions in this scenario?
- What would you do if this query needs to run every 5 minutes?
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 table of transactions that likely includes columns such as transaction_id, user_id, transaction_date, amount, and status. The goal is to produce a result set that efficiently aggr...
Approach
- Analyze the Existing Query: Start by examining the original query plan to identify bottlenecks, such as slow joins or inefficient aggregations.
- Use Common Table Expressions (CTEs): Break...
Submit Your Answer
Markdown supported