Optimize a slow query on impressions
Last updated: March 30, 2026
Quick Overview
A query on impressions is running slowly. Identify the bottleneck and optimize it.
Microsoft
March 30, 202645
12
675 solved
A query on impressions is running slowly. Identify the bottleneck and optimize it.
Data manipulation questions at Microsoft test your ability to work with real-world datasets. This Phone 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
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 this if the data was spread across multiple databases?
- How would you handle slowly changing dimensions in this scenario?
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Practice SQL ProblemsSample Answer
Problem Understanding
The query in question is focused on obtaining insights from a dataset containing 'impressions', which likely includes user interactions with advertisements or content. The key data fields involved may...
Approach
- Identify the JOINs: Determine if there are any unnecessary JOINs that can be eliminated or optimized.
- Review Aggregations: Look at the GROUP BY clauses to ensure that they are efficient...
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