Optimize a slow query on impressions
Last updated: August 25, 2025
Quick Overview
A query on impressions is running slowly. Identify the bottleneck and optimize it.
Datadog
August 25, 20252
5
3,464 solved
A query on impressions is running slowly. Identify the bottleneck and optimize it.
Datadog 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
- How would you handle slowly changing dimensions in this scenario?
- How would you validate the correctness of your query results?
- How would you handle this if the data was spread across multiple databases?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task involves optimizing a slow SQL query that retrieves data on impressions from a database. The main data sources likely include an impressions table that records various metrics about impress...
Approach
To optimize the query, I would follow these steps:
- Analyze the existing query: Identify parts that might be causing performance issues, such as nested subqueries or lack of indexing.
- **Use C...
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