Optimize a slow query on rides
Last updated: November 18, 2025
Quick Overview
A query on rides is running slowly. Identify the bottleneck and optimize it.
DoorDash
November 18, 2025551
8
1,063 solved
A query on rides is running slowly. Identify the bottleneck and optimize it.
DoorDash asks this during the Phone Screen 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
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?
- What indexes would you create to support this query?
- 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?
Sharpen Your Skills on Codemia
Practice similar problems with our interactive workspace, get AI feedback, and track your progress.
Practice SQL ProblemsSample Answer
Problem Understanding
The data involved in this query pertains to 'rides' in the DoorDash system, which likely consists of a table containing details about each ride, such as ride_id, driver_id, customer_id, start_time, en...
Approach
To optimize the slow query on rides, follow these steps:
- Identify the Current Query: Understand the existing SQL query and its execution plan to determine where the bottleneck is occurring. 2...
Submit Your Answer
