Optimize a slow query on products
Last updated: May 22, 2026
Quick Overview
A query on products is running slowly. Identify the bottleneck and optimize it.
Visa
Data Manipulation (SQL/Python)
Data Scientist
Visa
May 22, 2026Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Hard
139
5
2,416 solved
A query on products is running slowly. Identify the bottleneck and optimize it.
Data manipulation questions at Visa test your ability to work with real-world datasets. This Technical Screen 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
Pandas vectorized operations and groupby
Aggregate functions and GROUP BY
Subqueries and correlated subqueries
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
NULL handling and COALESCE
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 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?
- How would you handle this if the data was spread across multiple databases?
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Practice SQL ProblemsSample Answer
Problem Understanding
The query involves a dataset of products, which likely contains attributes such as product ID, name, category, price, and possibly sales data. The goal is to produce a result set that aggregates produ...
Approach
- Identify the existing query: Determine the current SQL query structure and which tables and columns it utilizes.
- Analyze execution plan: Use
EXPLAINto understand where the bottlenec...
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