Optimize a slow query on orders
Last updated: July 12, 2025
Quick Overview
A query on orders is running slowly. Identify the bottleneck and optimize it.
Grubhub
Data Manipulation (SQL/Python)
Data Scientist
Grubhub
July 12, 2025Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Medium
15
5
2,821 solved
A query on orders is running slowly. Identify the bottleneck and optimize it.
Data manipulation questions at Grubhub test your ability to work with real-world datasets. This Technical Screen 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
Index optimization and query performance
Aggregate functions and GROUP BY
Date/time manipulation
Data cleaning and transformation
Subqueries and correlated subqueries
JOIN types and when to use each
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?
- What would you do if this query needs to run every 5 minutes?
- What indexes would you create to support this query?
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Practice SQL ProblemsSample Answer
Problem Understanding
The problem involves a slow SQL query on the orders table in Grubhub's database. The orders table likely contains information about each order placed, including fields such as order_id, `custome...
Approach
- Identify the Query: First, determine the exact query that is running slowly. This may involve examining its structure, joins, and filters.
- Analyze Execution Plan: Use
EXPLAINto analyz...
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