Optimize a slow query on orders
Last updated: April 15, 2026
Quick Overview
A query on orders is running slowly. Identify the bottleneck and optimize it.
Jane Street
Data Manipulation (SQL/Python)
Data Scientist
Jane Street
April 15, 2026Data Scientist
Onsite
Data Manipulation (SQL/Python)
Easy
244
4
3,421 solved
A query on orders is running slowly. Identify the bottleneck and optimize it.
Jane Street 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
- Write syntactically correct SQL with proper JOIN and WHERE clauses
- Use GROUP BY and aggregate functions appropriately
- Handle NULL values correctly in your queries
- Explain the query execution plan at a high level
Key Topics to Cover
Pandas vectorized operations and groupby
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
JOIN types and when to use each
Subqueries and correlated subqueries
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 slowly changing dimensions in this scenario?
- What indexes would you create to support this query?
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Practice SQL ProblemsSample Answer
Problem Understanding
In this scenario, we are working with an 'orders' table that likely contains various columns such as 'order_id', 'customer_id', 'order_date', 'total_amount', and possibly 'status'. The goal is to iden...
Approach
- Examine the Original Query: Start by analyzing the existing SQL query to identify parts that may be causing slowness. Look for nested queries, inefficient joins, or absence of indexes.
- **Ide...
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