Optimize a slow query on clicks
Last updated: November 29, 2025
Quick Overview
A query on clicks is running slowly. Identify the bottleneck and optimize it.
Snapchat
November 29, 2025131
4
4,700 solved
A query on clicks is running slowly. Identify the bottleneck and optimize it.
Snapchat asks this during the Take-home Project 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 indexes would you create to support this query?
- Can you rewrite this without using subqueries?
- What would you do if this query needs to run every 5 minutes?
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Practice SQL ProblemsSample Answer
Problem Understanding
The query involves a table of 'clicks' that contains user interaction data with various attributes such as 'user_id', 'ad_id', 'timestamp', and 'click_value'. The goal is to produce a report showing t...
Approach
To optimize the query, I will follow these steps:
- Analyze the Current Query: Review the existing query structure to identify inefficient joins or aggregations.
- Identify Necessary Data: D...
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