Optimize a slow query on messages
Last updated: August 13, 2025
Quick Overview
A query on messages is running slowly. Identify the bottleneck and optimize it.
Anthropic
Data Manipulation (SQL/Python)
Data Scientist
Anthropic
August 13, 2025Data Scientist
Onsite
Data Manipulation (SQL/Python)
Medium
126
6
3,465 solved
A query on messages is running slowly. Identify the bottleneck and optimize it.
Anthropic 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
Aggregate functions and GROUP BY
Index optimization and query performance
Common Table Expressions (CTEs)
JOIN types and when to use each
Subqueries and correlated subqueries
Data cleaning and transformation
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 validate the correctness of your query results?
- 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 optimize this query for a table with 100 million rows?
Sharpen Your Skills on Codemia
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Practice SQL ProblemsSample Answer
Problem Understanding
The problem involves querying a database table named messages that contains records of messages sent in a communication platform. The objective is to retrieve and aggregate message data efficiently,...
Approach
- Identify Columns Needed: Determine which columns from the
messagestable are necessary for the query, such asuser_id,message_type,created_at, etc. - Analyze Current Query: Look ...
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