Optimize a slow query on impressions
Last updated: July 17, 2025
Quick Overview
A query on impressions is running slowly. Identify the bottleneck and optimize it.
Oracle
Data Manipulation (SQL/Python)
Data Scientist
Oracle
July 17, 2025Data Scientist
Onsite
Data Manipulation (SQL/Python)
Medium
8
10
4,457 solved
A query on impressions is running slowly. Identify the bottleneck and optimize it.
Data manipulation questions at Oracle test your ability to work with real-world datasets. This Onsite question evaluates your SQL proficiency, understanding of data modeling, and ability to derive insights from raw data.
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
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
Common Table Expressions (CTEs)
NULL handling and COALESCE
Date/time manipulation
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?
- What would you do if this query needs to run every 5 minutes?
- 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 involves a dataset of impressions, likely containing fields such as impression_id, user_id, ad_id, timestamp, and possibly other metrics related to ad performance. The goal is to analyze thi...
Approach
- Identify the Current Query: Review the existing SQL query to understand its structure and where it may be inefficient.
- Analyze Execution Plan: Use the EXPLAIN command to identify bottle...
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