Optimize a slow query on transactions
Last updated: March 17, 2026
Quick Overview
A query on transactions is running slowly. Identify the bottleneck and optimize it.
Robinhood
March 17, 202633
7
1,575 solved
A query on transactions is running slowly. Identify the bottleneck and optimize it.
Robinhood 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
- 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
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
- What would you do if this query needs to run every 5 minutes?
- How would you optimize this query for a table with 100 million rows?
- What indexes would you create to support this query?
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Practice SQL ProblemsSample Answer
Problem Understanding
The problem involves optimizing a slow SQL query that retrieves transaction data from the Robinhood database. The relevant data tables may include transactions, which likely contains columns such as...
Approach
To optimize the slow query, the following steps should be taken:
- Identify the Current Query: Start by analyzing the existing query to understand its structure, focusing on joins, filters, and a...
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